I have a small problem that I think should be easily handled by SQL Alchemy but I can't seem to get it right. I have two tables with one being a parent table and the other a child table. For each child record it needs a unique ID but only with the context of the unique parent record.

I am using the Declarative Base approach.

I setup the parent child relationship using the FKs and the relation function. What I'm trying to achieve is to get something like a pseudo autoincrement function that will lookup the max CategoryID value within the Category unique name group and increment it up one. I've tried using various default functions but the problem I run into is the inability to specify the CategoryUniqueName at the time of insertion. I can't find a way to pass the current value of CategoryItems.CategoryUniqueName so that the lookup query has the correct filter applied when trying to select something like func.max(CategoryItems.CategoryID). If I hardcode a query it works just fine. This is what I'm thinking should work but, again, I can't find a way to specify the unique value for the filter.

```
unique_group='my_group'
result=con.query(func.max(CategoryItems.CategoryID)).filter(and_(
        CategoryItems.CategoryUniqueName==unique_group, 
    )).one()

```

The classes are shown below. Much appreciate some guidance on how to accomplish this inside standard SQL Alchemy. I know I could always lookup the value and simply specify it directly within the same transaction but I'm trying to come up with a stand-alone SQL Alchemy approach that does not require additional logic elsewhere.

```
class Category(Base):
    __tablename__ = 'parent_table'
    __table_args__ = {'mysql_engine':'InnoDB', 'useexisting':True}

    CategoryUniqueName = Column(Unicode(255), primary_key=True)
    CategoryGroupName = Column(Unicode(255), nullable=False)
    CategoryGroupMemo = Column(UnicodeText)
    SortOrder = Column(Integer, index=True)
    IsLocked = Column(Boolean, default=0)

class CategoryItems(Base):
    __tablename__ = 'child_table'
    __table_args__ = {'mysql_engine':'InnoDB', 'useexisting':True}

    CategoryUniqueName = Column(Unicode(255), ForeignKey(Category.CategoryUniqueName), primary_key=True)
    CategoryID = Column(Integer, primary_key=True, autoincrement=False)
    CategoryName = Column(Unicode(255), nullable=False, index=True)
    CategoryMemo = Column(UnicodeText)
    CategoryImage = Column(Unicode(255))
    CategoryFlex1 = Column(Unicode(255), index=True)
    CategoryFlex2 = Column(Unicode(255), index=True)
    CategoryFlex3 = Column(Unicode(255), index=True)
    SortOrder = Column(Integer, index=True)

    category_group = relation(
        Category, 
        backref=backref(
            'items', 
            order_by=SortOrder, 
            collection_class=ordering_list('SortOrder'), 
            cascade="all, delete, delete-orphan"
    ))

```