Peewee: what does 'backref' really mean in foreign key?

Created on 28 Sep 2019 · 7Comments · Source: coleifer/peewee

I have read the documentation and try to understand the meaning of 'backref',which I have found in the word said "Every foreign-key field has an implied back-reference, which is exposed as a pre-filtered Select query using the provided backref attribute."But I still don't know how the back-reference could be exposed as a pre-filtered Select query,can you explain it more clearly please?

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bmxbmx3

👍6

Most helpful comment

It's pretty straightforward if you have an understanding of foreign-keys in the first place.

If you have a user and tweet model, the tweet has a foreign-key to user to indicate the author of the tweet:

class User(Model):
    username = TextField()

class Tweet(Model):
    user = ForeignKeyField(User, backref='tweets')
    content = TextField()

So when you create a tweet, you store the tweet content along with a reference to the user who created it. Thus a tweet has a relationship to one user, and a user has an implicit relationship to any number 0..n tweets.

The backref provides a way to access the tweets that refer to the given user.

user = User.create(username='huey')
Tweet.create(user=user, content='huey-1')
Tweet.create(user=user, content='huey-2')

# Get all tweets by huey.
query = Tweet.select().where(Tweet.user == user)

# Equivalent to above, using backref:
query = user.tweets

coleifer on 28 Sep 2019

👍9

All 7 comments

It's pretty straightforward if you have an understanding of foreign-keys in the first place.

If you have a user and tweet model, the tweet has a foreign-key to user to indicate the author of the tweet:

class User(Model):
    username = TextField()

class Tweet(Model):
    user = ForeignKeyField(User, backref='tweets')
    content = TextField()

The backref provides a way to access the tweets that refer to the given user.

user = User.create(username='huey')
Tweet.create(user=user, content='huey-1')
Tweet.create(user=user, content='huey-2')

# Get all tweets by huey.
query = Tweet.select().where(Tweet.user == user)

# Equivalent to above, using backref:
query = user.tweets

coleifer on 28 Sep 2019

👍9

It's pretty straightforward if you have an understanding of foreign-keys in the first place.

If you have a user and tweet model, the tweet has a foreign-key to user to indicate the author of the tweet:
class User(Model):
    username = TextField()

class Tweet(Model):
    user = ForeignKeyField(User, backref='tweets')
    content = TextField()
So when you create a tweet, you store the tweet content along with a reference to the user who created it. Thus a tweet has a relationship to _one_ user, and a user has an implicit relationship to any number 0..n tweets.

The backref provides a way to access the tweets that refer to the given user.
user = User.create(username='huey')
Tweet.create(user=user, content='huey-1')
Tweet.create(user=user, content='huey-2')

# Get all tweets by huey.
query = Tweet.select().where(Tweet.user == user)

# Equivalent to above, using backref:
query = user.tweets

Is this feature especially used for NoSQL?

bmxbmx3 on 28 Sep 2019

Is this feature especially used for NoSQL?

im-76a73f7f4d

coleifer on 28 Sep 2019

👍2

Is this feature especially used for NoSQL?

sorry,it seems that I've made a mistake.It's my fault...anyway,I just don't understand in your toy code why the object user could be the class variable of Tweet like this:
Tweet.create(user=**user**, content='huey-1')
why not the basic data types like int,char and so on?I haven't met it before,just seems a bit weird for me.

bmxbmx3 on 28 Sep 2019

Thank you! This is really useful😋