text: import re
  import datetime
  
  from django.db.migrations import operations
  from django.db.migrations.migration import Migration
  from django.db.migrations.questioner import MigrationQuestioner
  
  
  class MigrationAutodetector(object):
      """
      Takes a pair of ProjectStates, and compares them to see what the
      first would need doing to make it match the second (the second
      usually being the project's current state).
  
      Note that this naturally operates on entire projects at a time,
      as it's likely that changes interact (for example, you can't
      add a ForeignKey without having a migration to add the table it
      depends on first). A user interface may offer single-app usage
      if it wishes, with the caveat that it may not always be possible.
      """
  
      def __init__(self, from_state, to_state, questioner=None):
          self.from_state = from_state
          self.to_state = to_state
          self.questioner = questioner or MigrationQuestioner()
  
      def changes(self, graph, trim_to_apps=None):
          """
          Main entry point to produce a list of appliable changes.
          Takes a graph to base names on and an optional set of apps
          to try and restrict to (restriction is not guaranteed)
          """
          changes = self._detect_changes()
          changes = self._arrange_for_graph(changes, graph)
          if trim_to_apps:
              changes = self._trim_to_apps(changes, trim_to_apps)
          return changes
  
      def _detect_changes(self):
          """
          Returns a dict of migration plans which will achieve the
          change from from_state to to_state. The dict has app labels
          as keys and a list of migrations as values.
  
          The resulting migrations aren't specially named, but the names
          do matter for dependencies inside the set.
          """
          # We'll store migrations as lists by app names for now
          self.migrations = {}
          old_app_cache = self.from_state.render()
          new_app_cache = self.to_state.render()
          # Prepare lists of old/new model keys that we care about
          # (i.e. ignoring proxy ones)
          old_model_keys = [
              (al, mn)
              for al, mn in self.from_state.models.keys()
              if not old_app_cache.get_model(al, mn)._meta.proxy
          ]
          new_model_keys = [
              (al, mn)
              for al, mn in self.to_state.models.keys()
              if not new_app_cache.get_model(al, mn)._meta.proxy
          ]
          # Adding models. Phase 1 is adding models with no outward relationships.
          added_models = set(new_model_keys) - set(old_model_keys)
          pending_add = {}
          for app_label, model_name in added_models:
              model_state = self.to_state.models[app_label, model_name]
              # Are there any relationships out from this model? if so, punt it to the next phase.
              related_fields = []
              for field in new_app_cache.get_model(app_label, model_name)._meta.local_fields:
                  if field.rel:
                      if field.rel.to:
                          related_fields.append((field.name, field.rel.to._meta.app_label.lower(), field.rel.to._meta.object_name.lower()))
                      if hasattr(field.rel, "through") and not field.rel.though._meta.auto_created:
                          related_fields.append((field.name, field.rel.through._meta.app_label.lower(), field.rel.through._meta.object_name.lower()))
              if related_fields:
                  pending_add[app_label, model_name] = related_fields
              else:
                  self.add_to_migration(
                      app_label,
                      operations.CreateModel(
                          name=model_state.name,
                          fields=model_state.fields,
                          options=model_state.options,
                          bases=model_state.bases,
                      )
                  )
          # Phase 2 is progressively adding pending models, splitting up into two
          # migrations if required.
          pending_new_fks = []
          while pending_add:
              # Is there one we can add that has all dependencies satisfied?
              satisfied = [(m, rf) for m, rf in pending_add.items() if all((al, mn) not in pending_add for f, al, mn in rf)]
              if satisfied:
                  (app_label, model_name), related_fields = sorted(satisfied)[0]
                  model_state = self.to_state.models[app_label, model_name]
                  self.add_to_migration(
                      app_label,
                      operations.CreateModel(
                          name=model_state.name,
                          fields=model_state.fields,
                          options=model_state.options,
                          bases=model_state.bases,
                      )
                  )
                  for field_name, other_app_label, other_model_name in related_fields:
                      if app_label != other_app_label:
                          self.add_dependency(app_label, other_app_label)
                  del pending_add[app_label, model_name]
              # Ah well, we'll need to split one. Pick deterministically.
              else:
                  (app_label, model_name), related_fields = sorted(pending_add.items())[0]
                  model_state = self.to_state.models[app_label, model_name]
                  # Work out the fields that need splitting out
                  bad_fields = dict((f, (al, mn)) for f, al, mn in related_fields if (al, mn) in pending_add)
                  # Create the model, without those
                  self.add_to_migration(
                      app_label,
                      operations.CreateModel(
                          name=model_state.name,
                          fields=[(n, f) for n, f in model_state.fields if n not in bad_fields],
                          options=model_state.options,
                          bases=model_state.bases,
                      )
                  )
                  # Add the bad fields to be made in a phase 3
                  for field_name, (other_app_label, other_model_name) in bad_fields.items():
                      pending_new_fks.append((app_label, model_name, field_name, other_app_label))
                  del pending_add[app_label, model_name]
          # Phase 3 is adding the final set of FKs as separate new migrations
          for app_label, model_name, field_name, other_app_label in pending_new_fks:
              model_state = self.to_state.models[app_label, model_name]
              self.add_to_migration(
                  app_label,
                  operations.AddField(
                      model_name=model_name,
                      name=field_name,
                      field=model_state.get_field_by_name(field_name),
                  ),
                  new=True,
              )
              if app_label != other_app_label:
                  self.add_dependency(app_label, other_app_label)
          # Removing models
          removed_models = set(old_model_keys) - set(new_model_keys)
          for app_label, model_name in removed_models:
              model_state = self.from_state.models[app_label, model_name]
              self.add_to_migration(
                  app_label,
                  operations.DeleteModel(
                      model_state.name,
                  )
              )
          # Changes within models
          kept_models = set(old_model_keys).intersection(new_model_keys)
          old_fields = set()
          new_fields = set()
          for app_label, model_name in kept_models:
              old_model_state = self.from_state.models[app_label, model_name]
              new_model_state = self.to_state.models[app_label, model_name]
              # Collect field changes for later global dealing with (so AddFields
              # always come before AlterFields even on separate models)
              old_fields.update((app_label, model_name, x) for x, y in old_model_state.fields)
              new_fields.update((app_label, model_name, x) for x, y in new_model_state.fields)
              # Unique_together changes
              if old_model_state.options.get("unique_together", set()) != new_model_state.options.get("unique_together", set()):
                  self.add_to_migration(
                      app_label,
                      operations.AlterUniqueTogether(
                          name=model_name,
                          unique_together=new_model_state.options.get("unique_together", set()),
                      )
                  )
          # New fields
          for app_label, model_name, field_name in new_fields - old_fields:
              old_model_state = self.from_state.models[app_label, model_name]
              new_model_state = self.to_state.models[app_label, model_name]
              field = new_model_state.get_field_by_name(field_name)
              # Scan to see if this is actually a rename!
              field_dec = field.deconstruct()[1:]
              found_rename = False
              for rem_app_label, rem_model_name, rem_field_name in (old_fields - new_fields):
                  if rem_app_label == app_label and rem_model_name == model_name:
                      if old_model_state.get_field_by_name(rem_field_name).deconstruct()[1:] == field_dec:
                          if self.questioner.ask_rename(model_name, rem_field_name, field_name, field):
                              self.add_to_migration(
                                  app_label,
                                  operations.RenameField(
                                      model_name=model_name,
                                      old_name=rem_field_name,
                                      new_name=field_name,
                                  )
                              )
                              old_fields.remove((rem_app_label, rem_model_name, rem_field_name))
                              new_fields.remove((app_label, model_name, field_name))
                              found_rename = True
                              break
              if found_rename:
                  continue
              # You can't just add NOT NULL fields with no default
              if not field.null and not field.has_default():
                  field = field.clone()
                  field.default = self.questioner.ask_not_null_addition(field_name, model_name)
                  self.add_to_migration(
                      app_label,
                      operations.AddField(
                          model_name=model_name,
                          name=field_name,
                          field=field,
                          preserve_default=False,
                      )
                  )
              else:
                  self.add_to_migration(
                      app_label,
                      operations.AddField(
                          model_name=model_name,
                          name=field_name,
                          field=field,
                      )
                  )
          # Old fields
          for app_label, model_name, field_name in old_fields - new_fields:
              old_model_state = self.from_state.models[app_label, model_name]
              new_model_state = self.to_state.models[app_label, model_name]
              self.add_to_migration(
                  app_label,
                  operations.RemoveField(
                      model_name=model_name,
                      name=field_name,
                  )
              )
          # The same fields
          for app_label, model_name, field_name in old_fields.intersection(new_fields):
              # Did the field change?
              old_model_state = self.from_state.models[app_label, model_name]
              new_model_state = self.to_state.models[app_label, model_name]
              old_field_dec = old_model_state.get_field_by_name(field_name).deconstruct()
              new_field_dec = new_model_state.get_field_by_name(field_name).deconstruct()
              if old_field_dec != new_field_dec:
                  self.add_to_migration(
                      app_label,
                      operations.AlterField(
                          model_name=model_name,
                          name=field_name,
                          field=new_model_state.get_field_by_name(field_name),
                      )
                  )
          # Alright, now add internal dependencies
          for app_label, migrations in self.migrations.items():
              for m1, m2 in zip(migrations, migrations[1:]):
                  m2.dependencies.append((app_label, m1.name))
          # Clean up dependencies
          for app_label, migrations in self.migrations.items():
              for migration in migrations:
                  migration.dependencies = list(set(migration.dependencies))
          return self.migrations
  
      def add_to_migration(self, app_label, operation, new=False):
          migrations = self.migrations.setdefault(app_label, [])
          if not migrations or new:
              subclass = type("Migration", (Migration,), {"operations": [], "dependencies": []})
              instance = subclass("auto_%i" % (len(migrations) + 1), app_label)
              migrations.append(instance)
          migrations[-1].operations.append(operation)
  
      def add_dependency(self, app_label, other_app_label):
          """
          Adds a dependency to app_label's newest migration on
          other_app_label's latest migration.
          """
          if self.migrations.get(other_app_label, []):
              dependency = (other_app_label, self.migrations[other_app_label][-1].name)
          else:
              dependency = (other_app_label, "__first__")
          self.migrations[app_label][-1].dependencies.append(dependency)
  
      def _arrange_for_graph(self, changes, graph):
          """
          Takes in a result from changes() and a MigrationGraph,
          and fixes the names and dependencies of the changes so they
          extend the graph from the leaf nodes for each app.
          """
          leaves = graph.leaf_nodes()
          name_map = {}
          for app_label, migrations in list(changes.items()):
              if not migrations:
                  continue
              # Find the app label's current leaf node
              app_leaf = None
              for leaf in leaves:
                  if leaf[0] == app_label:
                      app_leaf = leaf
                      break
              # Do they want an initial migration for this app?
              if app_leaf is None and not self.questioner.ask_initial(app_label):
                  # They don't.
                  for migration in migrations:
                      name_map[(app_label, migration.name)] = (app_label, "__first__")
                  del changes[app_label]
              # Work out the next number in the sequence
              if app_leaf is None:
                  next_number = 1
              else:
                  next_number = (self.parse_number(app_leaf[1]) or 0) + 1
              # Name each migration
              for i, migration in enumerate(migrations):
                  if i == 0 and app_leaf:
                      migration.dependencies.append(app_leaf)
                  if i == 0 and not app_leaf:
                      new_name = "0001_initial"
                  else:
                      new_name = "%04i_%s" % (next_number, self.suggest_name(migration.operations))
                  name_map[(app_label, migration.name)] = (app_label, new_name)
                  migration.name = new_name
          # Now fix dependencies
          for app_label, migrations in changes.items():
              for migration in migrations:
                  migration.dependencies = [name_map.get(d, d) for d in migration.dependencies]
          return changes
  
      def _trim_to_apps(self, changes, app_labels):
          """
          Takes changes from arrange_for_graph and set of app labels and
          returns a modified set of changes which trims out as many migrations
          that are not in app_labels as possible.
          Note that some other migrations may still be present, as they may be
          required dependencies.
          """
          # Gather other app dependencies in a first pass
          app_dependencies = {}
          for app_label, migrations in changes.items():
              for migration in migrations:
                  for dep_app_label, name in migration.dependencies:
                      app_dependencies.setdefault(app_label, set()).add(dep_app_label)
          required_apps = set(app_labels)
          # Keep resolving till there's no change
          old_required_apps = None
          while old_required_apps != required_apps:
              old_required_apps = set(required_apps)
              for app_label in list(required_apps):
                  required_apps.update(app_dependencies.get(app_label, set()))
          # Remove all migrations that aren't needed
          for app_label in list(changes.keys()):
              if app_label not in required_apps:
                  del changes[app_label]
          return changes
  
      @classmethod
      def suggest_name(cls, ops):
          """
          Given a set of operations, suggests a name for the migration
          they might represent. Names are not guaranteed to be unique,
          but we put some effort in to the fallback name to avoid VCS conflicts
          if we can.
          """
          if len(ops) == 1:
              if isinstance(ops[0], operations.CreateModel):
                  return ops[0].name.lower()
              elif isinstance(ops[0], operations.DeleteModel):
                  return "delete_%s" % ops[0].name.lower()
              elif isinstance(ops[0], operations.AddField):
                  return "%s_%s" % (ops[0].model_name.lower(), ops[0].name.lower())
              elif isinstance(ops[0], operations.RemoveField):
                  return "remove_%s_%s" % (ops[0].model_name.lower(), ops[0].name.lower())
          elif all(isinstance(o, operations.CreateModel) for o in ops):
              return "_".join(sorted(o.name.lower() for o in ops))
          return "auto_%s" % datetime.datetime.now().strftime("%Y%m%d_%H%M")
  
      @classmethod
      def parse_number(cls, name):
          """
          Given a migration name, tries to extract a number from the
          beginning of it. If no number found, returns None.
          """
          if re.match(r"^\d+_", name):
              return int(name.split("_")[0])
          return None
  