Model Relationships

No Electronics Today!

In Review

You now know how to implement all of CRUD!

Web Apps in 4 Steps

Pick action (new, create, etc.)
Route to action (usu. use resources)
Write controller action
Create view if necessary

Now you know!

What file gets touched second when you type in 'localhost:3000/pokemons'?

```
app/views/pokemons/index.html.erb
```
```
app/controllers/pokemons_controller.rb
```
```
config/routes.rb
```
```
app/models/pokemon.rb
```

How do I query for all the pokemon that are above level 30?

```
Pokemon.where('level > 30')
```
```
Pokemon.where(level: '> 30')
```
```
Pokemon.find(30)
```
```
@pokemon.level(30)
```

Assume the Pokemon table has the level column as an integer.

users
id	name	num_badges
1	Sam	0
2	Mark	8

pokemons
id	name	level	description
1	Charmander	35	The best starter
4	Bulbasaur	25	The one no one chooses

How do I get the Pokemon that belongs to a User with name: "Sam" and id: 2?

```
Pokemon.find(User.find(2))
```
```
Pokemon.where(id: 2)
```
```
Pokemon.where(User.name: 'Sam')
```
```
Sam, this is impossible!
```

users
id	name	num_badges
1	Sam	0
2	Mark	8

pokemons
id	name	level	description
1	Charmander	5	The best starter
4	Bulbasaur	2	The one no one chooses

Houston, we have a problem!

pokemons
id	name	level	description
1	Charmander	5	The best starter
4	Bulbasaur	2	The one no one chooses

users
id	name	num_badges
1	Sam	0
2	Mark	8

How does a Pokemon know

which owner it belongs to?

Solution: a new column

pokemons
id	name	level	description
1	Charmander	5	The best starter
4	Bulbasaur	2	The one no one chooses

users
id	name	num_badges
1	Sam	0
2	Mark	8

pokemons
id	user_id	name	level	description
1	1	Charmander	5	The best starter
4	2	Bulbasaur	2	The one no one chooses

Key Idea: Model Associations

One model can belong to another
One model can have many of another

```
belongs_to in Pokemon
```
```
has_many in User
```

In Rails

Makes sense to ask a user for his/her pokemon:
```
@user.pokemons # [Pokemon(id: 2), ...]
```

Also makes sense to ask a pokemon for its user
```
@pokemon.user # User(id: 1)
```

Rails does this for us!

Checking the Docs

http://guides.rubyonrails.org/association_basics.html

You get a bunch of nifty methods just for including :has_many and :belongs_to!

```
@user.pokemons
```
```
@user.pokemons.build(...)
```
```
@user.pokemons.size
```

Which line errors?

@user = User.find(1)
@pokemon = Pokemon.find(1)
:belongs_to in Pokemon, :has_many in User

```
@pokemon.user
```
```
@user.pokemons
```
```
@user.pokemons.size
```
```
@user.pokemons.build(name: 'Ditto')
```

users
id	name	num_badges
1	Sam	0
2	Mark	8

pokemons
id	name	level	description
1	Charmander	5	The best starter
4	Bulbasaur	2	The one no one chooses

What Did I Forget?

Don't forget to actually make the user_id column!

How?

Let's do this.

Step 0

Initialize

Step 1

Migration

Step 2

Step 3

Model

Views

It's Lab Time.

github.com/rails-decal/fa15-lab6