Inheritance with relations and unique constraintss

Question

I’m trying to achieve polymorphism as described in SQLAlchemy docs.

I don’t really mind single table or joined table inheritance.

My constraints are

Child classes have references to direct parent classes, and only direct parent classes, not any class in the hierarchy: child -> parent, grand-child -> child. (This kind of breaks polymorphism when you think of it, if polymorphism means that any child class can be substituted to classes above in the hierarchy.)
Unique constraint on name (common attribute) + parent ID (defined in each subclass).

Joined table

This would allow me to define the relation from child to parent, but I don’t know how to represent the unique constraint on ("name", "parent_id") in Child.

class Parent(Base):
    __tablename__ = "parents"
    __mapper_args__ = {
        "polymorphic_on": "type_",
        "polymorphic_identity": "parent",
    }

    id = sqla.Column(sqla.Integer, primary_key=True)
    name = sqla.Column(sqla.String(80), nullable=False)
    type_ = sqla.Column(sqla.String(50))


class Child(Parent):
    __tablename__ = "children"
    __mapper_args__ = {
        "polymorphic_identity": "child",
    }

    id = sqla.Column(sqla.ForeignKey("parents.id"), primary_key=True)
    parent_id = sqla.Column(sqla.ForeignKey("parents.id"), nullable=False)

From this question, I must use a single table to enforce the constraint at DB level.

Single table

Here, I can define the unique constraint. But I don’t know how to restrict the relation to parent class to a specific type. E.g. ensure parent_id is a relation pointing to family table but only to rows of type parent.

class Parent(Base):
    __tablename__ = "family"
    __mapper_args__ = {
        "polymorphic_on": type_,
        "polymorphic_identity": "parent",
    }

    id = sqla.Column(sqla.Integer, primary_key=True)
    name = sqla.Column(sqla.String(80), nullable=False)


class Child(Parent):
    __table_args__ = (sqla.UniqueConstraint("parent_id", "name"),)
    __mapper_args__ = {
        "polymorphic_identity": "child",
    }

    parent_id = sqla.Column(sqla.ForeignKey("family.id"), nullable=False)

class OtherChild(Parent):
    __table_args__ = (sqla.UniqueConstraint("parent_id", "name"),)
    __mapper_args__ = {
        "polymorphic_identity": "other_child",
    }

    parent_id = sqla.Column(sqla.ForeignKey("family.id"), nullable=False)

How could I have both the relation and the unique constraint?

Asked By: Jérôme

||

Source

Answer 1

I came to the conclusion that the way around this is to duplicate name in the child tables.

class Parent(Base):
    __tablename__ = "parents"
    __mapper_args__ = {
        "polymorphic_on": "type_",
        "polymorphic_identity": "parent",
    }

    id = sqla.Column(sqla.Integer, primary_key=True)
    type_ = sqla.Column(sqla.String(50))


class Child(Parent):
    __tablename__ = "children"
    __table_args__ = (sqla.UniqueConstraint("parent_id", "name"),)
    __mapper_args__ = {
        "polymorphic_identity": "child",
    }

    id = sqla.Column(sqla.ForeignKey("parents.id"), primary_key=True)
    name = sqla.Column(sqla.String(80), nullable=False)
    parent_id = sqla.Column(sqla.ForeignKey("parents.id"), nullable=False)

class OtherChild(Parent):
    __tablename__ = "other_children"
    __table_args__ = (sqla.UniqueConstraint("parent_id", "name"),)
    __mapper_args__ = {
        "polymorphic_identity": "other_child",
    }

    id = sqla.Column(sqla.ForeignKey("parents.id"), primary_key=True)
    name = sqla.Column(sqla.String(80), nullable=False)
    parent_id = sqla.Column(sqla.ForeignKey("parents.id"), nullable=False)

My use case involves one parent table and multiple children tables, so this is duplication, but that’s the way it is.

I should add that this is not a SQLAlchemy limitation/issue, but rather due to how the data is stored in DB tables. I don’t think there can be a better way.

Answered By: Jérôme

Inheritance with relations and unique constraintss

Question:

Joined table

Single table

Answers: