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Now we're going to be learning about
another type of association that you'll

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probably be using more frequently
than has_and_belongs_to_many.

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It's called has_many_through.

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Here we have a new app with subscriber and
magazine models.

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We want people to be able to
subscribe to multiple magazines and

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we wanna be able to look up all
of the magazine subscribers.

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We could do this through a 
has_and_belongs_to_many association.

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We'd have a simply joined
table with subscriber_id and

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magazine_id call ups that could be
used to link subscribers to magazines.

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But what if we wanna track additional
info regarding the subscription itself,

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like how many months a person has
subscribed to a magazine for?

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has_and_belongs_to_many associations
can't do that sort of thing.

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Instead, we could set up a 
has_many_through association.

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has_many_through basically says that one
model is associated with many instances of

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another model through a third model.

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In the case,

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we could say that a subscriber has
many magazines through subscriptions.

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A subscription model instance joins
a subscriber to a magazine and

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a magazine to a subscriber.

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It works a lot like a 
has_and_belongs_to_many association.

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The table for
the subscriptions model has id fields for

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the two models it's joined,
subscribers and magazines.

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But in addition to the subscriber and
magazine id fields,

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it also has an id field of its own.

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Allowing you to look up subscriptions
independently of subscribers or magazines.

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[SOUND] And because a subscription
is a model on its own,

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it can also have additional attributes.

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Here we show an attribute that tracks the
number of months a subscription is for.

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In a has_many_through association,
two models are still joined by a database

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table but that table also
represents an entire third model.

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The first thing we're going to need to
do is generate the subscription model.

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So we'll run a bin/rails
generate model Subscription.

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We'll set up the subscription
zone attributes, so

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the number of months, that's an integer.

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And then we'll need to set up the foreign
keys that will associate the subscriber

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and the magazine together, so
subscriber_id is gonna be an integer.

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And that'll reference the id
column of the subscriber's table.

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And then we'll need to do the same for

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magazines, magazine_id,
that's also gonna be an integer.

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Then we'll need to go through and update
all three model classes with associations.

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So we'll go to the newly
generated subscription model.

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And we're going to say that it belongs_to
a subscriber,

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And it also belongs_to a magazine.

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That will allow us to
access the subscriber and

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the magazine if we look at
the subscription first.

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Then we'll need to go into
the Subscriber model class, and

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we're going to say that a subscriber
has_many :subscriptions.

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That'll allow us to look up the
subscriptions that belong to a subscriber.

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But we're also going to say
that it has_many :magazines

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through its subscriptions.

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This will add a magazines method to any
subscriber model instance that will look

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up all subscriptions the subscriber has.

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And then look up all magazines
that belong to those descriptions.

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We'll also need to do the same for
the Magazine model class.

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We'll say that magazine 
has_many :subscriptions.

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And it has_many :subscribers,

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through its subscriptions.

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Okay, let's save all that and

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go to our Rails console, and
try our new associations out.

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First, we'll create a new
subscription object.

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So we'll say,
subscription = Subscription class.create,

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we'll set its month attribute to 12, so
this will be a 12 month subscription.

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And we'll say the magazine
that the subscription is for

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is a result of Magazine class find_by,

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And we'll look a magazine up by title or
look up magazine titled Ruby Reader.

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I'm getting the error,
could not find table subscriptions and

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I know exactly what happened,
I forgot to run my migrations.

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So bin/rails db migrate.

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There we go,
now I'll run Rails console again.

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And previously, my call to
Subscription.create was unsuccessful

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because the table didn't exist.

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This time it should work.

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Yeah, there we go.

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So we've created a 12 month subscription
to the Ruby Reader magazine,

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now let's find the subscriber.

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Subscriber.find_by, Could have
find one with a name of Jay,

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And I'm going to add to that
subscriber's subscriptions collection.

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I'm gonna add the current
subscription to it.

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And that subscription should now be
associated with that subscriber.

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So let's pretty print Subscriber.find_by,
name: Jay.

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And let's print that subscriber's
collection of subscriptions.

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So there's that 12 month subscription.

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And the magazine itself should also
be associated with that subscriber

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through the subscription.

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So I'll pretty print
the subscriber Jay's magazines,

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And there's the Ruby Reader magazine.

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If we don't wanna create
a subscription manually,

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we can auto-create it by adding
a magazine direct to the subscriber.

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So let's say,
Subscriber.find_by(name: "Jay").

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magazines, and we'll append to that collection, Magazine.find_by

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title: iOS Inspired.

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And now if we look at that subscriber's
collection of subscriptions,

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we'll see a new automatically
created subscription on there.

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The auto-created join model won't have
its attributes set by default though.

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You'll notice that this
subscription is for nill months.

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So you'll need to update those
attributes if you want those set.

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We can say, Subscriber.find_by(name: "Jay").subscriptions.last.update,

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and we'll update its
months attribute to 6.

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Now if we print the list of subscriptions
again, we'll see that the last one's,

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months attribute has been set to 6.

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There's a lot more you can do with
has_many_through associations.

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See the teacher's notes
if you'd like more info.