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Active Record Associations in Rails

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Belongs To Associations

2:20 with Jay McGavren

We can easily look up the Comments that belong to a Post. But the way things are set up now, we might have a little more trouble looking up the Post that a Comment belongs to. The belongs_to association can fix that.

We can easily look up the Comments that belong to a Post. But the way things are set up now, we might have a little more trouble looking up the Post that a Comment belongs to.

  • Suppose all we had was a comment, and we wanted to get the Post that it belongs to.
    • Go to app/models/comment.rb, and add: belongs_to :post.
    • Now we can load up a Comment: comment = Comment.last, and access the Post it belongs to by calling its new post method: comment.post. We already made all the database changes we needed when adding the has_many :comments association to the Post class, so our belongs_to :post association works immediately.

Now you know how to set up belongs_to associations. Anytime you set up a has_many association from your first model to a second model, you're going to want to set up a belongs_to association from the second model back to the first.

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    We can easily look up the comments that belong to a post, but 0:00
    the way things are setup now, we might have a little more trouble looking up 0:03
    the post that a comment belongs to. 0:06
    So suppose that all we had was a comment and 0:09
    we wanted to get the post that it belongs to. 0:11
    We can say comment = Comment.last. 0:13
    Notice the post_id: 7 on the end there. 0:19
    That's from the foreign key column we added to the comments table. 0:23
    It's pretty easy to use that to do something 0:27
    like Post.find(comment.post_id). 0:32
    And there's Post id number 7 at the bottom. 0:38
    But it could be even easier. 0:41
    After all, we can get a post comments using a single method call like, 0:43
    Post.last.comments. 0:48
    Could we do something similar to get the post that belongs to a comment? 0:50
    Not at this point. 0:54
    If we try to call comment.post, we'll get NoMethodError: undefined method `post'. 0:55
    The only reason that we have a comments method on our post objects is that we 1:03
    added the has many comments association in the post model class. 1:07
    If we want a post method on our comment objects, 1:13
    we need to add a similar association in our comment class. 1:16
    So we'll go to app > models > comment.rb, and 1:20
    we'll add the association belongs_to :post. 1:24
    Again, and 1:28
    we use the symbol :post in all lower case instead of the name of the post class. 1:29
    Let's save this and go back to our console. 1:33
    And don't forget, we need to exit the console and 1:36
    then restart it with bin/rails c. 1:39
    That'll reload our code changes. 1:42
    Now we can load up a comment, comment = Comment.last. 1:44
    And we can access the post it belongs to by calling its new post method, 1:51
    comment.post. 1:54
    We already made all the database changes we needed when adding the has many 1:58
    comments association to the post class. 2:02
    So our belongs to post association works immediately. 2:04
    Now you know how to set up belongs to associations. 2:08
    Anytime you set up a has many association from your first model to a second model, 2:11
    you're going to want to set up a belongs to association from the second model 2:15
    back to the first. 2:19

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