SQLAlchemy 仅查找包含特定子代的父代

Question

I have the following many-to-many models:

class Association(db.Model):
    parent_id = db.Column(db.Integer, db.ForeignKey(
        'parent.id'), primary_key=True)
    child_id = db.Column(db.Integer, db.ForeignKey(
        'child.id'), primary_key=True)
    child = db.relationship("Child", back_populates="parents")
    parent = db.relationship("Parent", back_populates="children")


class Parent(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    children = db.relationship("Association", back_populates='parent', 
                               lazy='dynamic', cascade="save-update, merge, delete, delete-orphan")

class Child(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    parents = db.relationship("Association", back_populates='child', 
                              lazy='dynamic', cascade="save-update, merge, delete, delete-orphan")

How can I get all parents that have the exact children ids in a list (Example: [5,6]) and no additional ids?

Answer 1

If you have determined the children ids in a subquery, you can make a left join from association table to this subquery and check that the count is correct:

from sqlalchemy import func, and_

Values = [1 ,3]
childcount = len(Values)
childsquery = db.session.query(Child).filter(Child.id.in_(Values)).subquery()             
resultparents = db.session.query(Association.parent_id).outerjoin(childsquery).group_by(Association.parent_id).having(and_(func.count(Association.parent_id) == childcount, func.count(childsquery.c.id) ==  childcount))

Remark: I think your model needs to be corrected:

class Parent(db.Model):
   id = db.Column(db.Integer, primary_key=True)
   children = db.relationship("Association", back_populates='parent', 
            lazy='dynamic', cascade="save-update, merge, delete, delete-orphan")

class Child(db.Model):
   id = db.Column(db.Integer, primary_key=True)
   parents = db.relationship("Association", back_populates='child', 
                          lazy='dynamic', cascade="save-update, merge, delete, delete orphan")

Answer 2

I ended up solving it using the following code:

ids = [c1, c2, c3]
q1 = Parent.query.filter(Parent.children.any(
Association.child_id == c1)).filter(Parent.children.any(Association.child_id == c2))
.filter(Parent.children.any(Association.child_id == c3))
.filter(~Parent.children.any(Association.child_id.notin_(ids)))
.all()

But I am pretty sure there must be better ways to do this.

Answer 3

If you are using postgresql, the following should work:

from sqlalchemy.dialects.postgresql import array, array_agg
ids = [c1, c2, c3]
q1 = Parent.query.join(Child)
            .group_by(Parent)
            .having(
                array_agg(Child.id).contains(array(ids)), 
                count(Child.id) == len(ids)
            )

Most probably there will be alternatives for other databases as well.