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Model many-to-many relationships to build powerful Rails apps.

Associations II

So far we've made an app that stores data using two models. We used the `has_many` / `belongs_to` association to model a one-to-many relationship between the data.

But not all data is one-to-many. For example, a movie has many actors in the cast, but each actor also has many movies she's starred in. 

To model this data, we need a _many-to-many relationship_. Let's see how to do this by building a Rails app for a movie website.

Many to Many

Nice work! Let's review what we did:

1. We added columns to the movies table and actors table
2. We used the `t.belongs_to` methods in the parts table to add foreign keys, setting up the many-to-many relationship.
3. Finally we ran the migrations to update the database, and seeded the database with the data in **db/seeds.rb**.

Movies I

Great job! The app displays all actors that belong to a movie. The `has_many :through` association lets us easily query for all actors that belong to a movie.

Actors I

What did we just do? Check out the diagram in the browser:
1. We created three models - `Movie`, `Actor`, and `Part`. 
2. In the models, we used the `has_many :through` association to connect the `Movie` model to the `Actor` model through the `Part` model.

In this way, the `has_many :through` association sets up a many-to-many relationship between movies and actors.

Models II

Congratulations! You built a movie website that lets people browse a movie's cast and an actor's filmography. What can we generalize so far?
* Actors and movies can be modeled using a _many-to-many_ relationship. In Rails, this can be implemented using a `has_many :through` association.
* The `has_many :through` association joins two models via a third model.

Generalizations

Well done! The movies show up on the page.

Let's add another action to display a specific movie and its actors. Looking at the [seven standard controller actions](https://www.codecademy.com/articles/standard-controller-actions), we need to use the `show` action to do this.



Movies II

Looking at the request/response cycle, we need four parts to build the movie app: models, controllers, routes, and views.

Let's begin by creating the models.