父模型上的SQLAlchemy load_only [英] SQLAlchemy load_only on parent model

问题描述

当渴望加载子关系时，如何只加载父模型的几列:

When eager loading a child relationship, how can I only load few columns of the parent model:

如果我只需要chapters模型的title列，则此方法有效:

This works if I only need title column of the chapters model:

session.query(Book)\
    .options(joinedload('chapters').load_only('title'))

但这会引发错误:

session.query(Book.author)\
    .options(joinedload('chapters').load_only('title'))

查询仅具有基于表达式的实体-找不到名为 '章节'.

Query has only expression-based entities - can't find property named 'chapters'.

换句话说，我想要使用ORM语法的此SQL:

In other words I want this SQL in ORM syntax:

SELECT 
    books.author,
    chapters.title,
FROM 
    books JOIN chapters ON book.id = chapters.book_id

推荐答案

错误消息指出您仅选择Book.author，而不是Book的实例.如果chapters返回的只是字符串列表(对于author)，该去哪里?

The error message says you're only selecting Book.author, instead of instances of Book. Where is chapters going to go if all it's returning is a list of strings (for author).

您可以执行以下操作:

session.query(Book.author, Chapter.title).select_from(Book).join(Book.chapters)

或

session.query(Book).options(load_only("author"), joinedload("chapters").load_only("title"))

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