如何获得Flask-SQLAlchemy对象为Jinja模板加载关系子级?

问题描述

我有User和Post的基本模型.在我的用户模型中，我有

I have basic models for User and Post. In my User model, I have

posts = db.relationship('Post', backref='user', lazy='dynamic')

但是，当我做类似的事情

However, when I do something like

return render_template('user.html', users=users)

我想做类似的事情

{% for user in users %}
    <tr>
        <td>{{ user.id }}</td>
        <td>{{ user.posts|length }}</td>
    </tr>
{% endfor %}

不幸的是，这不起作用.帖子是查询，而不是lazy='dynamic'的对象b/c.如果更改lazy='joined'，我可以执行上述操作，但是无论何时我查询用户，它将为用户加载所有帖子.

Unfortunately, this doesn't work. Posts is a query, not an object b/c of lazy='dynamic'. I can do the above if I change lazy='joined', but then it would be loading all posts for users anytime I query for a User.

我尝试在查询中添加.options(joinedload('posts'))，但它表示

I tried adding .options(joinedload('posts')) to my query, but it said that

感谢您的帮助.

推荐答案

只需删除lazy="dynamic"参数，您就可以使用joinedload而不会出现问题. (请记住，执行此操作时会遇到难以诊断的N + 1查询问题.如果始终需要这些帖子，请改用joined).

Simply remove the lazy="dynamic" argument and you will be able to use a joinedload without issues. (Remember, when you do this you open yourself up to hard-to-diagnose N+1 query issues. If you will always need the posts, use joined instead).

如果您需要所有行为(可查询，可连接和延迟加载)，建议您查看，建议为动态查询添加另一个属性:

If you need all behaviors (queriable, joinable, and lazy-loaded), I suggest looking at the answer to this question, which suggests adding another attribute for dynamic queries:

class User(db.Model):
    posts = db.relationship('Post', backref='user')

class Post(db.Model):
    # etc.

User.posts_query = db.relationship(Post, lazy='dynamic')

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