Created
January 11, 2022 03:47
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Querying relationship count using SQLAlchemy
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| from sqlalchemy import Table, Integer, Column, ForeignKey, String, create_engine, select, func | |
| from sqlalchemy.orm import declarative_base, relationship, sessionmaker, column_property, undefer | |
| Base = declarative_base() | |
| engine = create_engine("sqlite://", echo=True) | |
| Session = sessionmaker(bind=engine, future=True) | |
| article_tags = Table( | |
| "article__tags", | |
| Base.metadata, | |
| Column( | |
| "tag_id", | |
| Integer, | |
| ForeignKey("tags.id"), | |
| index=True, | |
| primary_key=True, | |
| ), | |
| Column( | |
| "article_id", | |
| Integer, | |
| ForeignKey("articles.id"), | |
| primary_key=True, | |
| ), | |
| ) | |
| class Tag(Base): | |
| __tablename__ = "tags" | |
| id = Column(Integer, primary_key=True) | |
| name = Column(String(255), nullable=False, unique=True) | |
| articles = relationship("Article", secondary=article_tags, back_populates="tags") | |
| articles_count = column_property( | |
| select(func.count(article_tags.c.tag_id)).filter(article_tags.c.tag_id == id).scalar_subquery(), | |
| deferred=True, | |
| ) | |
| class Article(Base): | |
| __tablename__ = "articles" | |
| id = Column(Integer, primary_key=True) | |
| title = Column(String(length=255), nullable=False) | |
| tags = relationship("Tag", secondary=article_tags, back_populates="articles") | |
| def main(): | |
| Base.metadata.create_all(bind=engine) | |
| tag_py = Tag(name="Python") | |
| tag_db = Tag(name="Databases") | |
| with Session.begin() as session: | |
| session.add(Article(title="Article 1", tags=[tag_py])) | |
| session.add(Article(title="Article 2", tags=[tag_py])) | |
| session.add(Article(title="Article 3", tags=[tag_py])) | |
| session.add(Article(title="Article 4", tags=[tag_py, tag_db])) | |
| session.add(Article(title="Article 5", tags=[tag_py, tag_db])) | |
| # Using column_property | |
| print(select(Tag).options(undefer(Tag.articles_count))) | |
| # Making subquery manually (Exactly same the sql as when using column property) | |
| # But articles_count would not be present on Tag model | |
| subquery = select(func.count(article_tags.c.tag_id)).filter(Tag.id == article_tags.c.tag_id).scalar_subquery() | |
| query = select(subquery, Tag) | |
| print(query) | |
| # Using GroupBy | |
| query = select(article_tags.c.tag_id, func.count(article_tags.c.article_id)).group_by(article_tags.c.tag_id) | |
| for tag_id, articles_count in session.execute(query).all(): | |
| print(tag_id, articles_count) | |
| # Prints | |
| # 1 5 | |
| # 2 2 | |
| if __name__ == '__main__': | |
| main() |
Author
@zion-axas Haha, no problem, I don't even remember when and why I made this 😅
If you have a lot of entities and performance is critical I'd recommend to actually denormalize things a bit and keep a an article_count (in this example) if that's possible
Author
Denormalization would make things run a lot better, especially if you need to order by that count (e.g. we want to order our tags by amount of books in each)
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Thank you kind man for column_property(select(..))