What is belongs_to and has_many in Ruby on Rails
ActiveRecord makes it really easy to tie models in a Rails app to other models. In fact, you can even specify when you create a model what it should be associated with. If you already have to models and you would like one model to belong to the other, you’ll need to change both models and create a migration.
Let’s say you have a model, Tweet, and another model, User. A user can tweet many times,
and a tweet only belongs to one user. In ActiveRecord speak, User has_many tweets, and
a Tweet belongs_to a User. The database records are tied together, and you can build
features around the association easily.
How to Implement belongs_to and has_many
Continuing on the User/Tweet example, in the Tweet class, simply add belongs_to :user.
The class should look like this:
class Tweet < ApplicationRecord
belongs_to :user
end
In the User class, simply add has_many :tweets.
The class should look like this:
class Tweet < ApplicationRecord
has_many :tweets
end
This will tell rails to generate some methods you can use, but you still need to tie the database records together. Create a new migration by running:
rails generate migration AddUserIdToTweets
In the migration file, make the change method look like this:
def change
add_column :tweets, :user_id, :integer
end
This adds a user_id column to the Tweets database table, which rails will use
to keep track of which user a tweet object belongs to.
How to Get Use a belong_to and has_many relationship
You can use the association to do a number of things, including getting all
tweets that belong to a given user. The call is simple enough, and assuming
current_user refers to a User model, it looks like this:
current_user.tweets
To create a new Tweet that belong to a user, you can do this:
current_user.tweets.build(tweet_params)
Hope this was helpful!
