This course will be retired on June 1, 2025.
Bummer! You have been redirected as the page you requested could not be found.
Heads up! To view this whole video, sign in with your Courses account or enroll in your free 7-day trial. Sign In Enroll
Preview
Start a free Courses trial
to watch this video
Polymorphic belongs_to associations allow a model to belong to more than one type of other model.
You can set up a movie database app like we use in the video with these terminal commands:
rails new mdb
cd mdb
bin/rails g scaffold Movie title:string duration:integer
bin/rails g scaffold Show title:string episodes:integer
bin/rails g scaffold Actor name:string
bin/rails g migration AddProductionToActor production_id:integer production_type:string
bin/rails db:migrate
If you prefer, there's an alternate syntax for adding the _id and _type reference fields to the migration:
bin/rails g migration AddProductionToActor production:references{polymorphic}
Now we need to set up the polymorphic association in the model classes:
class Movie < ApplicationRecord
has_many :actors, as: :production
end
class Show < ApplicationRecord
has_many :actors, as: :production
end
class Actor < ApplicationRecord
belongs_to :production, polymorphic: true
end
With all that set up, you can add an Actor to either a Show or a Movie in the Rails console:
show = Show.create(title: "Game of Thrones")
show.actors
show.actors.create(name: "Peter Dinklage")
show.actors.create(name: "Lena Headey")
pp Show.first.actors
Actor.first.production
movie = Movie.create(title: "Fight Club")
movie.actors.create(name: "Brad Pitt")
movie.actors.create(name: "Edward Norton")
Movie.first.actors
pp Actor.all
You can read more about using polymorphic associations within your app here.
[/SCREENCAST]
Related Discussions
Have questions about this video? Start a discussion with the community and Treehouse staff.
Sign upRelated Discussions
Have questions about this video? Start a discussion with the community and Treehouse staff.
Sign up
[MUSIC]
0:00
When it comes down to it, associations
really just make some changes to
0:04
the SQL queries that Active Record
runs on your database.
0:08
If you want, Rails gives you the power
to take more control over those queries.
0:12
In this stage,
we'll look at some options for doing that.
0:16
Sometimes you'll have a model that
you want to belong to more than one
0:19
other model.
0:22
Both blog posts and
pages might have comments, for example, or
0:24
in a movie database both movies and
TV shows might have actors.
0:28
Supposed you had a movie
model that has many actors.
0:32
A movie ID field on the actors table
indicates which movie an actor belongs to.
0:35
But now you want to add a show
model to represent TV shows, and
0:41
you want to be able to associate
an actor with either a movie or a show.
0:44
So you decide to treat both movies and
shows as productions.
0:49
And you change the movie id field on
the actors table to a production id field
0:53
that can identify either a movie or
a show.
0:58
There's a problem though.
1:02
Based on the id alone,
1:03
Rails can't tell whether an actor
belongs to a movie or a show.
1:04
A production id of 3 could refer to
either the movie with an id of 3 or
1:08
the show with an id of 3.
1:12
To allow Active Record
to tell the difference,
1:15
we'll need to add a production
type field to the actors table.
1:17
It will hold one of two strings,
show if the actor belongs to a show, or
1:21
movie if the actor belongs to a movie.
1:26
Let's create a movie
database app to demonstrate.
1:29
I'm gonna change up a directory and
run rails new mdb.
1:32
Change into the new app directory,
and let's run bin/rails and
1:40
we're gonna generate a movie scaffold.
1:45
And we'll give movies two attributes,
1:53
a title that's a string, and
a duration that's an integer.
1:56
It'll be a number of seconds,
2:04
we'll multiply that to get
the number of hours and minutes.
2:05
That's not a detail we need to
worry about right now though.
2:08
Then I'll generate a second scaffold for
2:12
a Show model, that's also going to
have a title that has a string.
2:14
And we'll also track the number
of episodes in the show,
2:21
that'll be an integer.
2:24
And lastly, we'll generate a third
scaffold for an actor model.
2:28
That's going to have just one attribute
for now, a name that will be a string.
2:34
Now let's add the foreign key fields
that we're going to need to link
2:40
actors to productions.
2:43
So we'll run bin/rails generate migration,
2:45
and we'll call this migration
AddProductionToActor.
2:51
And we're gonna add two fields here,
the production_id,
2:58
which is just going to be an integer.
3:02
And by itself, this could identify
either a movie or a show.
3:06
To narrow it down, we're also going
to add a production_type field,
3:09
And that's going to be a string
that'll have the string movie or show.
3:15
And that will allow us to know which type
of model the production id refers to.
3:20
Okay, let's run all these
migrations we've generated.
3:27
bin/rails db:migrate, and
by the way, there's an alternate
3:29
syntax available for adding the id and
type reference fields to the migration.
3:35
See the teacher's notes for more info.
3:40
Okay, let's bring our
code up in an editor.
3:43
And now we're going to need to add
the associations to the model classes.
3:46
So first I'm going to go app models and
we'll edit the movie class.
3:50
And I'm gonna say that
a movie has_many :actors.
3:56
And this part is just like the has_many
declarations we made before, but
4:02
we're gonna add a new argument on here.
4:06
We're gonna say as and
the symbol production.
4:08
This will make it clear that Active Record
needs to use the production id and
4:13
production type fields to
look up the association.
4:17
I'll copy this line and we're gonna add it
exactly as is to the show class as well.
4:22
Lastly we need to edit
our actor model class.
4:32
And we need to specify
that an actor belongs to,
4:36
Not a movie or a show but a production.
4:41
And we need to add a second
argument saying, polymorphic: true.
4:46
This polymorphic flag here on
the end means that there’s going to
4:51
be many different models that could
take the form of the production.
4:56
Okay, let's save this and
5:03
go back to our terminal and try using
these associations in the Rails console.
5:04
So first I'm going to create a new show,
show = Show.create.
5:10
And I'll give a title, Game of Thrones.
5:19
At first our show won't have
any actors associated with it,
5:26
although there's
Active Method we can call.
5:29
So let's try adding some actors now,
show.actors.create,
5:32
and this one will have
a name of Peter Dinklage.
5:40
And let's add a sector actor
with the name of Lena Headey,
5:48
And now if we pretty print
the first show's actors,
5:54
There are both the actors we added.
6:01
We can also link from an actor back
to the production it belongs to.
6:04
So if we say Actor.first.production,
6:07
let's pretty print that, we see that
it bring up the show Game of Thrones.
6:13
Now let's try creating a movie.
6:18
movie = Movie.create, and
we'll give it a title of Fight Club.
6:20
And let's add a couple
actors to this as well,
6:33
movie.actors.create.
6:38
First one's name is going to be Brad Pitt.
6:42
Then we'll create a second actor
with the name of Edward Norton.
6:47
Again, belonging to this
second production, this movie.
6:52
And again we can bring up the list
of actors through the association
6:57
on the movie.
7:00
Movie.first.actors, let's
pretty print that,
7:01
and there they are,
Brad Pitt and Edward Norton.
7:07
We can also pretty print
the list of all actors, Whoops,
7:09
I pluralized my class name.
7:15
Shouldn't have done that.
7:17
One second.
Pretty print actor.all.
7:18
And there's the list of all actors,
7:22
regardless of whether they
belong to to a movie or a show.
7:23
So that's polymorphic associations.
7:26
You won't be using them all the time, but
when you need them they're really useful.
7:28
See the teacher's notes if you'd like more
info about using polymorphic associations
7:32
within your app.
7:36
You need to sign up for Treehouse in order to download course files.
Sign upYou need to sign up for Treehouse in order to set up Workspace
Sign up