Detailed example of python flask many-to-many table query

When we study flask, we will inevitably encounter many-to-many table query. I also encountered this problem today. Let me share my ideas to the Script Home platform for your reference

When we are learning flask, we will inevitably encounter many-to-many table queries. I also encountered this problem today. Then I thought about it for a long time. I didn't think of a solution. I tried several methods, but maybe I gave up due to limitations of my thinking. Later, I went to Baidu online, but found that there was still a certain gap between the results from Baidu and what I wanted. So I Based on the ideas I got from Baidu, I also explored my data structure. Let's take a look at how I query it here. First, I will show you a fragment of the database code I wrote, so as to deepen your understanding.

post_class=db.Table('post_class',
  db.Column('post_id',db.Integer(),db.ForeignKey('posts.id')),
  db.Column('classifa_id',db.Integer(),db.ForeignKey('fenlei.id')))
class Post(db.Model):#文章表
  tablename='posts'
  id=db.Column(db.Integer,primary_key=True,autoincrement=True)
  title=db.Column(db.String(255),unique=True)
  text=db.Column(db.Text())
  publish_date=db.Column(db.DateTime,default=datetime.datetime.now())
  user_id=db.Column(db.Integer,db.ForeignKey('users.id'))
  is_recomment=db.Column(db.Boolean,default=False)
  comments = db.relationship(
    'Comment',
    backref='posts',
    lazy='dynamic')
  tag = db.relationship(
    'Tag',
    secondary=posts_tags,
    backref=db.backref('posts', lazy='dynamic')
  )
  classname=db.relationship('Classifa',
    secondary=post_class,
    backref=db.backref('posts'))
  def repr(self):
    return "<Model Post `{}`>".format(self.title)
class Classifa(db.Model):#分类
  tablename=&#39;fenlei&#39;
  id=db.Column(db.Integer(),primary_key=True)
  name=db.Column(db.String(64))
  def repr(self):
    return self.name

There are three tables here. One is a list of articles, and the other is a classification table. Let’s think about it. An article may belong to multiple A category, then a category may also belong to multiple articles. In this way, we must all understand this logic. So, my third table shows the many-to-many relationship, so how do we query it next? In fact, My current need is to find all the articles under a category.

Let’s take a look at my code

data=Classifa.query.filter_by(name='数据库').first()
 data_post=data.posts

Here, I Directly find this category from the category, and then query the articles belonging to this category through the third table. In fact, it is very simple here. Maybe it was because my brain was short-circuited at the time. I don’t know what I thought was right. Looking at it now, it is actually It was still so simple, but I overlooked something at the time. Come on, learn on the way forward.

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