SQLAlchemy / Flask - Get number of rows in relationship table

Question

Here's my scenario. I have a table of Books each book has a table of Questions and each question has a table of Answers.

What I want to do is efficiently get the amount of answers for a given list of Question.id. Example:

# 6,000 Question.id's for Book.id == 1
questions = [0, 1, 2, 3, 4, 5, 6, ..., 6000]

Here's what I'm doing and its proving to be very very inefficient:

# This takes a couple minutes for it to finish
count = 0
query = QuestionModel.query.filter(QuestionModel.id.in_(questions)).all()

for q in query:
  count += len(list(q.Answers))

# count = 3456

I'm assuming this is so slow because q.Answers is actually getting populated with data.

Here are my models:

class BookModel(db.base):
    __tablename__ = 'books_book'
    __table_args__ = {
        'autoload': True,
        'extend_existing': True,
        'autoload_with': db.instance.engine,
    }
    id = Column(Integer, primary_key=True)
    Chapters = relationship(ChapterModel)
    QuestionMeta = relationship(QuestionMetaModel)

class QuestionMetaModel(db.base):
    __tablename__ = 'questions_questionmeta'
    __table_args__ = {
        'autoload': True,
        'extend_existing': True,
        'autoload_with': db.instance.engine,
    }
    id = Column(Integer, primary_key=True)
    question_id = Column(ForeignKey('questions_question.id'))
    book_id = Column(ForeignKey('books_book.id'))
    chapter_id = Column(ForeignKey('books_chapter.id'))

class QuestionModel(db.base):
    __tablename__ = 'questions_question'
    __table_args__ = {
        'autoload': True,
        'extend_existing': True,
        'autoload_with': db.instance.engine,
    }
    id = Column(Integer, primary_key=True)
    Answers = relationship(AnswerModel)

class AnswerModel(db.base):
    __tablename__ = 'answers_answer'
    __table_args__ = {
        'autoload': True,
        'extend_existing': True,
        'autoload_with': db.instance.engine,
    }
    id = Column(Integer, primary_key=True)
    question_id = Column(ForeignKey('questions_question.id'))

Question: All I want is the amount of entries in QuestionModel.Answers not the actual data itself. How would I accomplish this, so it doesn't take 2 minutes for one Book.id? Grabbing the Question.id for each book is fast, but grabbing the amount of answers for each Question.id is very slow.

Answer 1

Perform a join across BookModel, QuestionModel, and AnswerModel and use SQLAlchemy func.count.

from sqlalchemy import func

count_query = (session.query(BookModel, QuestionModel, AnswerModel, func.count(AnswerModel.id))
    .select_from(AnswerModel)
    .join(QuestionModel)
    .join(BookModel)
    .group_by(BookModel)
)