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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]
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[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
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