Introduction

We want to use many-to-many relationships to allow our posts to be "liked" by other users. The reason this is a many-to-many relationship is because:

• A Post can have many Users that liked the photo
• A User can have many Posts that they've liked

The end goal of this is that we should be able to ask for:

@photo = ...
@photo.users_liked # => Should return a list of Users who liked this photo!

@user = ...
@user.posts_liked # => Should return a list of Photos this user liked!

Getting started

Let's begin by running a generator to create our Like model (this gets run in your terminal):

rails generate model Like user:references post:references

Remember to migrate your database after this runs, to update your table structure with the new Like model.

Take a look at our Like model that was generated to see that it is a "junction" or "join" model between Post and User (by belonging to both models).

Setting up relations

Next, we'll add two has_many relations to our Post model:

class Post
	has_many :likes
	has_many :users_liked, through: :likes, source: :user
end

🧐What's with this source business?

The reason why this code looks a bit different than the Book and Author is because we want to be able access users who liked this post with @post.users_liked instead of @post.users (it's more descriptive!).

Rails was able to imply which field we wanted from Authorship between Books and Authors because we were accessing it as @book.authors and @author.books, but if we want to call it something else, we have to be a bit more explicit, as above, with source.

Next, try adding the reverse has_many to the User model. It should look very similar to the has_many above.

• Solution