text: # -*- coding: utf-8 -
  import hashlib
  import operator
  
  from django.db.backends.creation import BaseDatabaseCreation
  from django.db.backends.utils import truncate_name
  from django.db.models.fields.related import ManyToManyField
  from django.db.transaction import atomic
  from django.utils.encoding import force_bytes
  from django.utils.log import getLogger
  from django.utils.six.moves import reduce
  from django.utils import six
  
  logger = getLogger('django.db.backends.schema')
  
  
  def _related_objects(old_field, new_field):
      # Returns (old_relation, new_relation) tuples.
      return zip(
          old_field.model._meta.get_all_related_objects(),
          new_field.model._meta.get_all_related_objects()
      )
  
  
  class BaseDatabaseSchemaEditor(object):
      """
      This class (and its subclasses) are responsible for emitting schema-changing
      statements to the databases - model creation/removal/alteration, field
      renaming, index fiddling, and so on.
  
      It is intended to eventually completely replace DatabaseCreation.
  
      This class should be used by creating an instance for each set of schema
      changes (e.g. a syncdb run, a migration file), and by first calling start(),
      then the relevant actions, and then commit(). This is necessary to allow
      things like circular foreign key references - FKs will only be created once
      commit() is called.
      """
  
      # Overrideable SQL templates
      sql_create_table = "CREATE TABLE %(table)s (%(definition)s)"
      sql_create_table_unique = "UNIQUE (%(columns)s)"
      sql_rename_table = "ALTER TABLE %(old_table)s RENAME TO %(new_table)s"
      sql_retablespace_table = "ALTER TABLE %(table)s SET TABLESPACE %(new_tablespace)s"
      sql_delete_table = "DROP TABLE %(table)s CASCADE"
  
      sql_create_column = "ALTER TABLE %(table)s ADD COLUMN %(column)s %(definition)s"
      sql_alter_column = "ALTER TABLE %(table)s %(changes)s"
      sql_alter_column_type = "ALTER COLUMN %(column)s TYPE %(type)s"
      sql_alter_column_null = "ALTER COLUMN %(column)s DROP NOT NULL"
      sql_alter_column_not_null = "ALTER COLUMN %(column)s SET NOT NULL"
      sql_alter_column_default = "ALTER COLUMN %(column)s SET DEFAULT %(default)s"
      sql_alter_column_no_default = "ALTER COLUMN %(column)s DROP DEFAULT"
      sql_delete_column = "ALTER TABLE %(table)s DROP COLUMN %(column)s CASCADE"
      sql_rename_column = "ALTER TABLE %(table)s RENAME COLUMN %(old_column)s TO %(new_column)s"
      sql_update_with_default = "UPDATE %(table)s SET %(column)s = %(default)s WHERE %(column)s IS NULL"
  
      sql_create_check = "ALTER TABLE %(table)s ADD CONSTRAINT %(name)s CHECK (%(check)s)"
      sql_delete_check = "ALTER TABLE %(table)s DROP CONSTRAINT %(name)s"
  
      sql_create_unique = "ALTER TABLE %(table)s ADD CONSTRAINT %(name)s UNIQUE (%(columns)s)"
      sql_delete_unique = "ALTER TABLE %(table)s DROP CONSTRAINT %(name)s"
  
      sql_create_fk = "ALTER TABLE %(table)s ADD CONSTRAINT %(name)s FOREIGN KEY (%(column)s) REFERENCES %(to_table)s (%(to_column)s) DEFERRABLE INITIALLY DEFERRED"
      sql_create_inline_fk = None
      sql_delete_fk = "ALTER TABLE %(table)s DROP CONSTRAINT %(name)s"
  
      sql_create_index = "CREATE INDEX %(name)s ON %(table)s (%(columns)s)%(extra)s"
      sql_delete_index = "DROP INDEX %(name)s"
  
      sql_create_pk = "ALTER TABLE %(table)s ADD CONSTRAINT %(name)s PRIMARY KEY (%(columns)s)"
      sql_delete_pk = "ALTER TABLE %(table)s DROP CONSTRAINT %(name)s"
  
      def __init__(self, connection, collect_sql=False):
          self.connection = connection
          self.collect_sql = collect_sql
          if self.collect_sql:
              self.collected_sql = []
  
      # State-managing methods
  
      def __enter__(self):
          self.deferred_sql = []
          if self.connection.features.can_rollback_ddl:
              self.atomic = atomic(self.connection.alias)
              self.atomic.__enter__()
          return self
  
      def __exit__(self, exc_type, exc_value, traceback):
          if exc_type is None:
              for sql in self.deferred_sql:
                  self.execute(sql)
          if self.connection.features.can_rollback_ddl:
              self.atomic.__exit__(exc_type, exc_value, traceback)
  
      # Core utility functions
  
      def execute(self, sql, params=[]):
          """
          Executes the given SQL statement, with optional parameters.
          """
          # Log the command we're running, then run it
          logger.debug("%s; (params %r)" % (sql, params))
          if self.collect_sql:
              ending = "" if sql.endswith(";") else ";"
              if params is not None:
                  self.collected_sql.append((sql % tuple(map(self.quote_value, params))) + ending)
              else:
                  self.collected_sql.append(sql + ending)
          else:
              with self.connection.cursor() as cursor:
                  cursor.execute(sql, params)
  
      def quote_name(self, name):
          return self.connection.ops.quote_name(name)
  
      # Field <-> database mapping functions
  
      def column_sql(self, model, field, include_default=False):
          """
          Takes a field and returns its column definition.
          The field must already have had set_attributes_from_name called.
          """
          # Get the column's type and use that as the basis of the SQL
          db_params = field.db_parameters(connection=self.connection)
          sql = db_params['type']
          params = []
          # Check for fields that aren't actually columns (e.g. M2M)
          if sql is None:
              return None, None
          # Work out nullability
          null = field.null
          # If we were told to include a default value, do so
          default_value = self.effective_default(field)
          include_default = include_default and not self.skip_default(field)
          if include_default and default_value is not None:
              if self.connection.features.requires_literal_defaults:
                  # Some databases can't take defaults as a parameter (oracle)
                  # If this is the case, the individual schema backend should
                  # implement prepare_default
                  sql += " DEFAULT %s" % self.prepare_default(default_value)
              else:
                  sql += " DEFAULT %s"
                  params += [default_value]
          # Oracle treats the empty string ('') as null, so coerce the null
          # option whenever '' is a possible value.
          if (field.empty_strings_allowed and not field.primary_key and
                  self.connection.features.interprets_empty_strings_as_nulls):
              null = True
          if null and not self.connection.features.implied_column_null:
              sql += " NULL"
          elif not null:
              sql += " NOT NULL"
          # Primary key/unique outputs
          if field.primary_key:
              sql += " PRIMARY KEY"
          elif field.unique:
              sql += " UNIQUE"
          # Optionally add the tablespace if it's an implicitly indexed column
          tablespace = field.db_tablespace or model._meta.db_tablespace
          if tablespace and self.connection.features.supports_tablespaces and field.unique:
              sql += " %s" % self.connection.ops.tablespace_sql(tablespace, inline=True)
          # Return the sql
          return sql, params
  
      def skip_default(self, field):
          """
          Some backends don't accept default values for certain columns types
          (i.e. MySQL longtext and longblob).
          """
          return False
  
      def prepare_default(self, value):
          """
          Only used for backends which have requires_literal_defaults feature
          """
          raise NotImplementedError('subclasses of BaseDatabaseSchemaEditor for backends which have requires_literal_defaults must provide a prepare_default() method')
  
      def effective_default(self, field):
          """
          Returns a field's effective database default value
          """
          if field.has_default():
              default = field.get_default()
          elif not field.null and field.blank and field.empty_strings_allowed:
              if field.get_internal_type() == "BinaryField":
                  default = six.binary_type()
              else:
                  default = six.text_type()
          else:
              default = None
          # If it's a callable, call it
          if six.callable(default):
              default = default()
          # Run it through the field's get_db_prep_save method so we can send it
          # to the database.
          default = field.get_db_prep_save(default, self.connection)
          return default
  
      def quote_value(self, value):
          """
          Returns a quoted version of the value so it's safe to use in an SQL
          string. This is not safe against injection from user code; it is
          intended only for use in making SQL scripts or preparing default values
          for particularly tricky backends (defaults are not user-defined, though,
          so this is safe).
          """
          raise NotImplementedError()
  
      # Actions
  
      def create_model(self, model):
          """
          Takes a model and creates a table for it in the database.
          Will also create any accompanying indexes or unique constraints.
          """
          # Create column SQL, add FK deferreds if needed
          column_sqls = []
          params = []
          for field in model._meta.local_fields:
              # SQL
              definition, extra_params = self.column_sql(model, field)
              if definition is None:
                  continue
              # Check constraints can go on the column SQL here
              db_params = field.db_parameters(connection=self.connection)
              if db_params['check']:
                  definition += " CHECK (%s)" % db_params['check']
              # Autoincrement SQL (for backends with inline variant)
              col_type_suffix = field.db_type_suffix(connection=self.connection)
              if col_type_suffix:
                  definition += " %s" % col_type_suffix
              params.extend(extra_params)
              # FK
              if field.rel and field.db_constraint:
                  to_table = field.rel.to._meta.db_table
                  to_column = field.rel.to._meta.get_field(field.rel.field_name).column
                  if self.connection.features.supports_foreign_keys:
                      self.deferred_sql.append(self._create_fk_sql(model, field, "_fk_%(to_table)s_%(to_column)s"))
                  elif self.sql_create_inline_fk:
                      definition += " " + self.sql_create_inline_fk % {
                          "to_table": self.quote_name(to_table),
                          "to_column": self.quote_name(to_column),
                      }
              # Add the SQL to our big list
              column_sqls.append("%s %s" % (
                  self.quote_name(field.column),
                  definition,
              ))
              # Autoincrement SQL (for backends with post table definition variant)
              if field.get_internal_type() == "AutoField":
                  autoinc_sql = self.connection.ops.autoinc_sql(model._meta.db_table, field.column)
                  if autoinc_sql:
                      self.deferred_sql.extend(autoinc_sql)
          # Add any unique_togethers
          for fields in model._meta.unique_together:
              columns = [model._meta.get_field_by_name(field)[0].column for field in fields]
              column_sqls.append(self.sql_create_table_unique % {
                  "columns": ", ".join(self.quote_name(column) for column in columns),
              })
          # Make the table
          sql = self.sql_create_table % {
              "table": self.quote_name(model._meta.db_table),
              "definition": ", ".join(column_sqls)
          }
          if model._meta.db_tablespace:
              tablespace_sql = self.connection.ops.tablespace_sql(model._meta.db_tablespace)
              if tablespace_sql:
                  sql += ' ' + tablespace_sql
  
          self.execute(sql, params)
  
          # Add any field index and index_together's (deferred as SQLite3 _remake_table needs it)
          self.deferred_sql.extend(self._model_indexes_sql(model))
  
          # Make M2M tables
          for field in model._meta.local_many_to_many:
              if field.rel.through._meta.auto_created:
                  self.create_model(field.rel.through)
  
      def delete_model(self, model):
          """
          Deletes a model from the database.
          """
          # Handle auto-created intermediary models
          for field in model._meta.local_many_to_many:
              if field.rel.through._meta.auto_created:
                  self.delete_model(field.rel.through)
  
          # Delete the table
          self.execute(self.sql_delete_table % {
              "table": self.quote_name(model._meta.db_table),
          })
  
      def alter_unique_together(self, model, old_unique_together, new_unique_together):
          """
          Deals with a model changing its unique_together.
          Note: The input unique_togethers must be doubly-nested, not the single-
          nested ["foo", "bar"] format.
          """
          olds = set(tuple(fields) for fields in old_unique_together)
          news = set(tuple(fields) for fields in new_unique_together)
          # Deleted uniques
          for fields in olds.difference(news):
              columns = [model._meta.get_field_by_name(field)[0].column for field in fields]
              constraint_names = self._constraint_names(model, columns, unique=True)
              if len(constraint_names) != 1:
                  raise ValueError("Found wrong number (%s) of constraints for %s(%s)" % (
                      len(constraint_names),
                      model._meta.db_table,
                      ", ".join(columns),
                  ))
              self.execute(self._delete_constraint_sql(self.sql_delete_unique, model, constraint_names[0]))
          # Created uniques
          for fields in news.difference(olds):
              columns = [model._meta.get_field_by_name(field)[0].column for field in fields]
              self.execute(self._create_unique_sql(model, columns))
  
      def alter_index_together(self, model, old_index_together, new_index_together):
          """
          Deals with a model changing its index_together.
          Note: The input index_togethers must be doubly-nested, not the single-
          nested ["foo", "bar"] format.
          """
          olds = set(tuple(fields) for fields in old_index_together)
          news = set(tuple(fields) for fields in new_index_together)
          # Deleted indexes
          for fields in olds.difference(news):
              columns = [model._meta.get_field_by_name(field)[0].column for field in fields]
              constraint_names = self._constraint_names(model, list(columns), index=True)
              if len(constraint_names) != 1:
                  raise ValueError("Found wrong number (%s) of constraints for %s(%s)" % (
                      len(constraint_names),
                      model._meta.db_table,
                      ", ".join(columns),
                  ))
              self.execute(self._delete_constraint_sql(self.sql_delete_index, model, constraint_names[0]))
          # Created indexes
          for field_names in news.difference(olds):
              fields = [model._meta.get_field_by_name(field)[0] for field in field_names]
              self.execute(self._create_index_sql(model, fields, suffix="_idx"))
  
      def alter_db_table(self, model, old_db_table, new_db_table):
          """
          Renames the table a model points to.
          """
          if old_db_table == new_db_table:
              return
          self.execute(self.sql_rename_table % {
              "old_table": self.quote_name(old_db_table),
              "new_table": self.quote_name(new_db_table),
          })
  
      def alter_db_tablespace(self, model, old_db_tablespace, new_db_tablespace):
          """
          Moves a model's table between tablespaces
          """
          self.execute(self.sql_retablespace_table % {
              "table": self.quote_name(model._meta.db_table),
              "old_tablespace": self.quote_name(old_db_tablespace),
              "new_tablespace": self.quote_name(new_db_tablespace),
          })
  
      def add_field(self, model, field):
          """
          Creates a field on a model.
          Usually involves adding a column, but may involve adding a
          table instead (for M2M fields)
          """
          # Special-case implicit M2M tables
          if ((isinstance(field, ManyToManyField) or field.get_internal_type() == 'ManyToManyField') and
                  field.rel.through._meta.auto_created):
              return self.create_model(field.rel.through)
          # Get the column's definition
          definition, params = self.column_sql(model, field, include_default=True)
          # It might not actually have a column behind it
          if definition is None:
              return
          # Check constraints can go on the column SQL here
          db_params = field.db_parameters(connection=self.connection)
          if db_params['check']:
              definition += " CHECK (%s)" % db_params['check']
          # Build the SQL and run it
          sql = self.sql_create_column % {
              "table": self.quote_name(model._meta.db_table),
              "column": self.quote_name(field.column),
              "definition": definition,
          }
          self.execute(sql, params)
          # Drop the default if we need to
          # (Django usually does not use in-database defaults)
          if not self.skip_default(field) and field.default is not None:
              sql = self.sql_alter_column % {
                  "table": self.quote_name(model._meta.db_table),
                  "changes": self.sql_alter_column_no_default % {
                      "column": self.quote_name(field.column),
                  }
              }
              self.execute(sql)
          # Add an index, if required
          if field.db_index and not field.unique:
              self.deferred_sql.append(self._create_index_sql(model, [field]))
          # Add any FK constraints later
          if field.rel and self.connection.features.supports_foreign_keys and field.db_constraint:
              self.deferred_sql.append(self._create_fk_sql(model, field, "_fk_%(to_table)s_%(to_column)s"))
          # Reset connection if required
          if self.connection.features.connection_persists_old_columns:
              self.connection.close()
  
      def remove_field(self, model, field):
          """
          Removes a field from a model. Usually involves deleting a column,
          but for M2Ms may involve deleting a table.
          """
          # Special-case implicit M2M tables
          if ((isinstance(field, ManyToManyField) or field.get_internal_type() == 'ManyToManyField') and
                  field.rel.through._meta.auto_created):
              return self.delete_model(field.rel.through)
          # It might not actually have a column behind it
          if field.db_parameters(connection=self.connection)['type'] is None:
              return
          # Drop any FK constraints, MySQL requires explicit deletion
          if field.rel:
              fk_names = self._constraint_names(model, [field.column], foreign_key=True)
              for fk_name in fk_names:
                  self.execute(self._delete_constraint_sql(self.sql_delete_fk, model, fk_name))
          # Delete the column
          sql = self.sql_delete_column % {
              "table": self.quote_name(model._meta.db_table),
              "column": self.quote_name(field.column),
          }
          self.execute(sql)
          # Reset connection if required
          if self.connection.features.connection_persists_old_columns:
              self.connection.close()
  
      def alter_field(self, model, old_field, new_field, strict=False):
          """
          Allows a field's type, uniqueness, nullability, default, column,
          constraints etc. to be modified.
          Requires a copy of the old field as well so we can only perform
          changes that are required.
          If strict is true, raises errors if the old column does not match old_field precisely.
          """
          # Ensure this field is even column-based
          old_db_params = old_field.db_parameters(connection=self.connection)
          old_type = old_db_params['type']
          new_db_params = new_field.db_parameters(connection=self.connection)
          new_type = new_db_params['type']
          if ((old_type is None and old_field.rel is None) or
                  (new_type is None and new_field.rel is None)):
              raise ValueError(
                  "Cannot alter field %s into %s - they do not properly define "
                  "db_type (are you using PostGIS 1.5 or badly-written custom "
                  "fields?)" % (old_field, new_field)
              )
          elif old_type is None and new_type is None and (
                  old_field.rel.through and new_field.rel.through and
                  old_field.rel.through._meta.auto_created and
                  new_field.rel.through._meta.auto_created):
              return self._alter_many_to_many(model, old_field, new_field, strict)
          elif old_type is None and new_type is None and (old_field.rel.through and new_field.rel.through and not old_field.rel.through._meta.auto_created and not new_field.rel.through._meta.auto_created):
              # Both sides have through models; this is a no-op.
              return
          elif old_type is None or new_type is None:
              raise ValueError("Cannot alter field %s into %s - they are not compatible types (you cannot alter to or from M2M fields, or add or remove through= on M2M fields)" % (
                  old_field,
                  new_field,
              ))
  
          self._alter_field(model, old_field, new_field, old_type, new_type, old_db_params, new_db_params, strict)
  
      def _alter_field(self, model, old_field, new_field, old_type, new_type, old_db_params, new_db_params, strict=False):
          """Actually perform a "physical" (non-ManyToMany) field update."""
  
          # Drop any FK constraints, we'll remake them later
          fks_dropped = set()
          if old_field.rel and old_field.db_constraint:
              fk_names = self._constraint_names(model, [old_field.column], foreign_key=True)
              if strict and len(fk_names) != 1:
                  raise ValueError("Found wrong number (%s) of foreign key constraints for %s.%s" % (
                      len(fk_names),
                      model._meta.db_table,
                      old_field.column,
                  ))
              for fk_name in fk_names:
                  fks_dropped.add((old_field.column,))
                  self.execute(self._delete_constraint_sql(self.sql_delete_fk, model, fk_name))
          # Has unique been removed?
          if old_field.unique and (not new_field.unique or (not old_field.primary_key and new_field.primary_key)):
              # Find the unique constraint for this field
              constraint_names = self._constraint_names(model, [old_field.column], unique=True)
              if strict and len(constraint_names) != 1:
                  raise ValueError("Found wrong number (%s) of unique constraints for %s.%s" % (
                      len(constraint_names),
                      model._meta.db_table,
                      old_field.column,
                  ))
              for constraint_name in constraint_names:
                  self.execute(self._delete_constraint_sql(self.sql_delete_unique, model, constraint_name))
          # Drop incoming FK constraints if we're a primary key and things are going
          # to change.
          if old_field.primary_key and new_field.primary_key and old_type != new_type:
              for _old_rel, new_rel in _related_objects(old_field, new_field):
                  rel_fk_names = self._constraint_names(
                      new_rel.model, [new_rel.field.column], foreign_key=True
                  )
                  for fk_name in rel_fk_names:
                      self.execute(self._delete_constraint_sql(self.sql_delete_fk, new_rel.model, fk_name))
          # Removed an index? (no strict check, as multiple indexes are possible)
          if (old_field.db_index and not new_field.db_index and
                  not old_field.unique and not
                  (not new_field.unique and old_field.unique)):
              # Find the index for this field
              index_names = self._constraint_names(model, [old_field.column], index=True)
              for index_name in index_names:
                  self.execute(self._delete_constraint_sql(self.sql_delete_index, model, index_name))
          # Change check constraints?
          if old_db_params['check'] != new_db_params['check'] and old_db_params['check']:
              constraint_names = self._constraint_names(model, [old_field.column], check=True)
              if strict and len(constraint_names) != 1:
                  raise ValueError("Found wrong number (%s) of check constraints for %s.%s" % (
                      len(constraint_names),
                      model._meta.db_table,
                      old_field.column,
                  ))
              for constraint_name in constraint_names:
                  self.execute(self._delete_constraint_sql(self.sql_delete_check, model, constraint_name))
          # Have they renamed the column?
          if old_field.column != new_field.column:
              self.execute(self.sql_rename_column % {
                  "table": self.quote_name(model._meta.db_table),
                  "old_column": self.quote_name(old_field.column),
                  "new_column": self.quote_name(new_field.column),
                  "type": new_type,
              })
          # Next, start accumulating actions to do
          actions = []
          null_actions = []
          post_actions = []
          # Type change?
          if old_type != new_type:
              fragment, other_actions = self._alter_column_type_sql(
                  model._meta.db_table, old_field, new_field, new_type
              )
              actions.append(fragment)
              post_actions.extend(other_actions)
          # When changing a column NULL constraint to NOT NULL with a given
          # default value, we need to perform 4 steps:
          #  1. Add a default for new incoming writes
          #  2. Update existing NULL rows with new default
          #  3. Replace NULL constraint with NOT NULL
          #  4. Drop the default again.
          # Default change?
          old_default = self.effective_default(old_field)
          new_default = self.effective_default(new_field)
          needs_database_default = (
              old_default != new_default and
              new_default is not None and
              not self.skip_default(new_field)
          )
          if needs_database_default:
              if self.connection.features.requires_literal_defaults:
                  # Some databases can't take defaults as a parameter (oracle)
                  # If this is the case, the individual schema backend should
                  # implement prepare_default
                  actions.append((
                      self.sql_alter_column_default % {
                          "column": self.quote_name(new_field.column),
                          "default": self.prepare_default(new_default),
                      },
                      [],
                  ))
              else:
                  actions.append((
                      self.sql_alter_column_default % {
                          "column": self.quote_name(new_field.column),
                          "default": "%s",
                      },
                      [new_default],
                  ))
          # Nullability change?
          if old_field.null != new_field.null:
              if new_field.null:
                  null_actions.append((
                      self.sql_alter_column_null % {
                          "column": self.quote_name(new_field.column),
                          "type": new_type,
                      },
                      [],
                  ))
              else:
                  null_actions.append((
                      self.sql_alter_column_not_null % {
                          "column": self.quote_name(new_field.column),
                          "type": new_type,
                      },
                      [],
                  ))
          # Only if we have a default and there is a change from NULL to NOT NULL
          four_way_default_alteration = (
              new_field.has_default() and
              (old_field.null and not new_field.null)
          )
          if actions or null_actions:
              if not four_way_default_alteration:
                  # If we don't have to do a 4-way default alteration we can
                  # directly run a (NOT) NULL alteration
                  actions = actions + null_actions
              # Combine actions together if we can (e.g. postgres)
              if self.connection.features.supports_combined_alters and actions:
                  sql, params = tuple(zip(*actions))
                  actions = [(", ".join(sql), reduce(operator.add, params))]
              # Apply those actions
              for sql, params in actions:
                  self.execute(
                      self.sql_alter_column % {
                          "table": self.quote_name(model._meta.db_table),
                          "changes": sql,
                      },
                      params,
                  )
              if four_way_default_alteration:
                  # Update existing rows with default value
                  self.execute(
                      self.sql_update_with_default % {
                          "table": self.quote_name(model._meta.db_table),
                          "column": self.quote_name(new_field.column),
                          "default": "%s",
                      },
                      [new_default],
                  )
                  # Since we didn't run a NOT NULL change before we need to do it
                  # now
                  for sql, params in null_actions:
                      self.execute(
                          self.sql_alter_column % {
                              "table": self.quote_name(model._meta.db_table),
                              "changes": sql,
                          },
                          params,
                      )
          if post_actions:
              for sql, params in post_actions:
                  self.execute(sql, params)
          # Added a unique?
          if not old_field.unique and new_field.unique:
              self.execute(self._create_unique_sql(model, [new_field.column]))
          # Added an index?
          if (not old_field.db_index and new_field.db_index and
                  not new_field.unique and not
                  (not old_field.unique and new_field.unique)):
              self.execute(self._create_index_sql(model, [new_field], suffix="_uniq"))
          # Type alteration on primary key? Then we need to alter the column
          # referring to us.
          rels_to_update = []
          if old_field.primary_key and new_field.primary_key and old_type != new_type:
              rels_to_update.extend(_related_objects(old_field, new_field))
          # Changed to become primary key?
          # Note that we don't detect unsetting of a PK, as we assume another field
          # will always come along and replace it.
          if not old_field.primary_key and new_field.primary_key:
              # First, drop the old PK
              constraint_names = self._constraint_names(model, primary_key=True)
              if strict and len(constraint_names) != 1:
                  raise ValueError("Found wrong number (%s) of PK constraints for %s" % (
                      len(constraint_names),
                      model._meta.db_table,
                  ))
              for constraint_name in constraint_names:
                  self.execute(self._delete_constraint_sql(self.sql_delete_pk, model, constraint_name))
              # Make the new one
              self.execute(
                  self.sql_create_pk % {
                      "table": self.quote_name(model._meta.db_table),
                      "name": self.quote_name(self._create_index_name(model, [new_field.column], suffix="_pk")),
                      "columns": self.quote_name(new_field.column),
                  }
              )
              # Update all referencing columns
              rels_to_update.extend(_related_objects(old_field, new_field))
          # Handle our type alters on the other end of rels from the PK stuff above
          for old_rel, new_rel in rels_to_update:
              rel_db_params = new_rel.field.db_parameters(connection=self.connection)
              rel_type = rel_db_params['type']
              fragment, other_actions = self._alter_column_type_sql(
                  new_rel.model._meta.db_table, old_rel.field, new_rel.field, rel_type
              )
              self.execute(
                  self.sql_alter_column % {
                      "table": self.quote_name(new_rel.model._meta.db_table),
                      "changes": fragment[0],
                  },
                  fragment[1],
              )
              for sql, params in other_actions:
                  self.execute(sql, params)
          # Does it have a foreign key?
          if (new_field.rel and
                  (fks_dropped or not old_field.rel or not old_field.db_constraint) and
                  new_field.db_constraint):
              self.execute(self._create_fk_sql(model, new_field, "_fk_%(to_table)s_%(to_column)s"))
          # Rebuild FKs that pointed to us if we previously had to drop them
          if old_field.primary_key and new_field.primary_key and old_type != new_type:
              for rel in new_field.model._meta.get_all_related_objects():
                  self.execute(self._create_fk_sql(rel.model, rel.field, "_fk"))
          # Does it have check constraints we need to add?
          if old_db_params['check'] != new_db_params['check'] and new_db_params['check']:
              self.execute(
                  self.sql_create_check % {
                      "table": self.quote_name(model._meta.db_table),
                      "name": self.quote_name(self._create_index_name(model, [new_field.column], suffix="_check")),
                      "column": self.quote_name(new_field.column),
                      "check": new_db_params['check'],
                  }
              )
          # Drop the default if we need to
          # (Django usually does not use in-database defaults)
          if needs_database_default:
              sql = self.sql_alter_column % {
                  "table": self.quote_name(model._meta.db_table),
                  "changes": self.sql_alter_column_no_default % {
                      "column": self.quote_name(new_field.column),
                  }
              }
              self.execute(sql)
          # Reset connection if required
          if self.connection.features.connection_persists_old_columns:
              self.connection.close()
  
      def _alter_column_type_sql(self, table, old_field, new_field, new_type):
          """
          Hook to specialize column type alteration for different backends,
          for cases when a creation type is different to an alteration type
          (e.g. SERIAL in PostgreSQL, PostGIS fields).
  
          Should return two things; an SQL fragment of (sql, params) to insert
          into an ALTER TABLE statement, and a list of extra (sql, params) tuples
          to run once the field is altered.
          """
          return (
              (
                  self.sql_alter_column_type % {
                      "column": self.quote_name(new_field.column),
                      "type": new_type,
                  },
                  [],
              ),
              [],
          )
  
      def _alter_many_to_many(self, model, old_field, new_field, strict):
          """
          Alters M2Ms to repoint their to= endpoints.
          """
          # Rename the through table
          if old_field.rel.through._meta.db_table != new_field.rel.through._meta.db_table:
              self.alter_db_table(old_field.rel.through, old_field.rel.through._meta.db_table, new_field.rel.through._meta.db_table)
          # Repoint the FK to the other side
          self.alter_field(
              new_field.rel.through,
              # We need the field that points to the target model, so we can tell alter_field to change it -
              # this is m2m_reverse_field_name() (as opposed to m2m_field_name, which points to our model)
              old_field.rel.through._meta.get_field_by_name(old_field.m2m_reverse_field_name())[0],
              new_field.rel.through._meta.get_field_by_name(new_field.m2m_reverse_field_name())[0],
          )
          self.alter_field(
              new_field.rel.through,
              # for self-referential models we need to alter field from the other end too
              old_field.rel.through._meta.get_field_by_name(old_field.m2m_field_name())[0],
              new_field.rel.through._meta.get_field_by_name(new_field.m2m_field_name())[0],
          )
  
      def _create_index_name(self, model, column_names, suffix=""):
          """
          Generates a unique name for an index/unique constraint.
          """
          # If there is just one column in the index, use a default algorithm from Django
          if len(column_names) == 1 and not suffix:
              return truncate_name(
                  '%s_%s' % (model._meta.db_table, BaseDatabaseCreation._digest(column_names[0])),
                  self.connection.ops.max_name_length()
              )
          # Else generate the name for the index using a different algorithm
          table_name = model._meta.db_table.replace('"', '').replace('.', '_')
          index_unique_name = '_%x' % abs(hash((table_name, ','.join(column_names))))
          max_length = self.connection.ops.max_name_length() or 200
          # If the index name is too long, truncate it
          index_name = ('%s_%s%s%s' % (table_name, column_names[0], index_unique_name, suffix)).replace('"', '').replace('.', '_')
          if len(index_name) > max_length:
              part = ('_%s%s%s' % (column_names[0], index_unique_name, suffix))
              index_name = '%s%s' % (table_name[:(max_length - len(part))], part)
          # It shouldn't start with an underscore (Oracle hates this)
          if index_name[0] == "_":
              index_name = index_name[1:]
          # If it's STILL too long, just hash it down
          if len(index_name) > max_length:
              index_name = hashlib.md5(force_bytes(index_name)).hexdigest()[:max_length]
          # It can't start with a number on Oracle, so prepend D if we need to
          if index_name[0].isdigit():
              index_name = "D%s" % index_name[:-1]
          return index_name
  
      def _create_index_sql(self, model, fields, suffix="", sql=None):
          """
          Return the SQL statement to create the index for one or several fields.
          `sql` can be specified if the syntax differs from the standard (GIS
          indexes, ...).
          """
          if len(fields) == 1 and fields[0].db_tablespace:
              tablespace_sql = self.connection.ops.tablespace_sql(fields[0].db_tablespace)
          elif model._meta.db_tablespace:
              tablespace_sql = self.connection.ops.tablespace_sql(model._meta.db_tablespace)
          else:
              tablespace_sql = ""
          if tablespace_sql:
              tablespace_sql = " " + tablespace_sql
  
          columns = [field.column for field in fields]
          sql_create_index = sql or self.sql_create_index
          return sql_create_index % {
              "table": self.quote_name(model._meta.db_table),
              "name": self.quote_name(self._create_index_name(model, columns, suffix=suffix)),
              "columns": ", ".join(self.quote_name(column) for column in columns),
              "extra": tablespace_sql,
          }
  
      def _model_indexes_sql(self, model):
          """
          Return all index SQL statements (field indexes, index_together) for the
          specified model, as a list.
          """
          if not model._meta.managed or model._meta.proxy or model._meta.swapped:
              return []
          output = []
          for field in model._meta.local_fields:
              if field.db_index and not field.unique:
                  output.append(self._create_index_sql(model, [field], suffix=""))
  
          for field_names in model._meta.index_together:
              fields = [model._meta.get_field_by_name(field)[0] for field in field_names]
              output.append(self._create_index_sql(model, fields, suffix="_idx"))
          return output
  
      def _create_fk_sql(self, model, field, suffix):
          from_table = model._meta.db_table
          from_column = field.column
          to_table = field.related_field.model._meta.db_table
          to_column = field.related_field.column
          suffix = suffix % {
              "to_table": to_table,
              "to_column": to_column,
          }
  
          return self.sql_create_fk % {
              "table": self.quote_name(from_table),
              "name": self.quote_name(self._create_index_name(model, [from_column], suffix=suffix)),
              "column": self.quote_name(from_column),
              "to_table": self.quote_name(to_table),
              "to_column": self.quote_name(to_column),
          }
  
      def _create_unique_sql(self, model, columns):
          return self.sql_create_unique % {
              "table": self.quote_name(model._meta.db_table),
              "name": self.quote_name(self._create_index_name(model, columns, suffix="_uniq")),
              "columns": ", ".join(self.quote_name(column) for column in columns),
          }
  
      def _delete_constraint_sql(self, template, model, name):
          return template % {
              "table": self.quote_name(model._meta.db_table),
              "name": self.quote_name(name),
          }
  
      def _constraint_names(self, model, column_names=None, unique=None,
                            primary_key=None, index=None, foreign_key=None,
                            check=None):
          """
          Returns all constraint names matching the columns and conditions
          """
          column_names = list(column_names) if column_names else None
          with self.connection.cursor() as cursor:
              constraints = self.connection.introspection.get_constraints(cursor, model._meta.db_table)
          result = []
          for name, infodict in constraints.items():
              if column_names is None or column_names == infodict['columns']:
                  if unique is not None and infodict['unique'] != unique:
                      continue
                  if primary_key is not None and infodict['primary_key'] != primary_key:
                      continue
                  if index is not None and infodict['index'] != index:
                      continue
                  if check is not None and infodict['check'] != check:
                      continue
                  if foreign_key is not None and not infodict['foreign_key']:
                      continue
                  result.append(name)
          return result
  