Question

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"
    ))

Answer 1

I see 3 ways to go:

1. The most obvious and well documented. Create a mapper extension with before_insert() hook replacing inserted parameter.
2. Pass function as default argument. This function is called with context parameter with all data you need: context.compiled_parameters[0]['CategoryUniqueName'], context.connection.
3. Pass FetchedValue() in server_default parameter and use trigger to do the job server side.

All these solutions have race condition mentioned by ddaa. In case of race condition your code won’t break database state, but will fail with exception when primary key is defined properly (it’s not true for your code!). It might be acceptable for some application to fail (show 500 page in web application) in some rare cases.

Note, that you have defined a CategoryID as primary key. This won’t allow reuse of the same number for different values of CategoryUniqueName column. You have to change it to composite primary index for 2 columns.