sqlalchemy 按孩子月/年计算唯一父母

Question

This is kind of hard for me to explain but I'm trying to create a query to count how many unique parents there are by month depending on if they have children with birth dates in that month. So I have a parent and child model that have a relationship. Each child has a birthdate and parent id and each parent can have multiple children within the same month of their other children. If a Parent has 10 children in the month of January, I only want them to be counted once. I can count how many children have birthdates by month like so:

children_query = db.query(func.count(Child.id).label('Children'),
                            func.extract('year', Child.birth_date),
                            func.extract('month', Child.birth_date))\
                            .group_by(func.extract('year', Child.birth_date),
                                    func.extract('month', Child.birth_date)).all()

But im having trouble wrapping my head around creating a query to count how many parents have children with birthdates in each month. Basically the result I need needs to show there are 10 unique parents with children in Jan, 11 in Feb, 5 in Mar, etc..

Is there a way to remove duplicate parent_ids and count by parent_id?

Here are the models

class Parent(Base):
    id = Column(Integer, primary_key=True)
    children = relationship('Child', backref="parent")

class Child(Base):
    id = Column(Integer, primary_key=True)
    parent_id = Column(Integer, ForeignKey('parent.id'))
    birth_date = Column(DateTime, nullable=False)

Answer 1

I think this query does what you want:

q = (session.query(sa.func.extract('YEAR', Child.birth_date).label('year'),
                   sa.func.extract('MONTH', Child.birth_date).label('month'),
                   sa.func.count(sa.func.distinct(Child.parent_id)))
            .group_by('year', 'month')
            .order_by('year', 'month'))

it generates this SQL (on Postgres)

SELECT EXTRACT(YEAR FROM children.birth_date) AS year, 
       EXTRACT(MONTH FROM children.birth_date) AS month,
       count(distinct(children.parent_id)) AS count_1 
  FROM children 
  GROUP BY year, month 
  ORDER BY year, month

The trick is using COUNT(DISTINCT...) .