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Add migration example to Association Basics [ci skip]

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Ryan Castillo 2012-12-05 20:22:12 -05:00
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@ -94,6 +94,25 @@ end
NOTE: `belongs_to` associations _must_ use the singular term. If you used the pluralized form in the above example for the `customer` association in the `Order` model, you would be told that there was an "uninitialized constant Order::Customers". This is because Rails automatically infers the class name from the association name. If the association name is wrongly pluralized, then the inferred class will be wrongly pluralized too.
The corresponding migration might look like this:
```ruby
class CreateOrders < ActiveRecord::Migration
def change
create_table :customers do |t|
t.string :name
t.timestamps
end
create_table :orders do |t|
t.belongs_to :customer
t.datetime :order_date
t.timestamps
end
end
end
```
### The `has_one` Association
A `has_one` association also sets up a one-to-one connection with another model, but with somewhat different semantics (and consequences). This association indicates that each instance of a model contains or possesses one instance of another model. For example, if each supplier in your application has only one account, you'd declare the supplier model like this:
@ -106,6 +125,25 @@ end
![has_one Association Diagram](images/has_one.png)
The corresponding migration might look like this:
```ruby
class CreateSuppliers < ActiveRecord::Migration
def change
create_table :suppliers do |t|
t.string :name
t.timestamps
end
create_table :accounts do |t|
t.belongs_to :supplier
t.string :account_number
t.timestamps
end
end
end
```
### The `has_many` Association
A `has_many` association indicates a one-to-many connection with another model. You'll often find this association on the "other side" of a `belongs_to` association. This association indicates that each instance of the model has zero or more instances of another model. For example, in an application containing customers and orders, the customer model could be declared like this:
@ -120,6 +158,25 @@ NOTE: The name of the other model is pluralized when declaring a `has_many` asso
![has_many Association Diagram](images/has_many.png)
The corresponding migration might look like this:
```ruby
class CreateCustomers < ActiveRecord::Migration
def change
create_table :customers do |t|
t.string :name
t.timestamps
end
create_table :orders do |t|
t.belongs_to :customer
t.datetime :order_date
t.timestamps
end
end
end
```
### The `has_many :through` Association
A `has_many :through` association is often used to set up a many-to-many connection with another model. This association indicates that the declaring model can be matched with zero or more instances of another model by proceeding _through_ a third model. For example, consider a medical practice where patients make appointments to see physicians. The relevant association declarations could look like this:
@ -143,6 +200,31 @@ end
![has_many :through Association Diagram](images/has_many_through.png)
The corresponding migration might look like this:
```ruby
class CreateAppointments < ActiveRecord::Migration
def change
create_table :physicians do |t|
t.string :name
t.timestamps
end
create_table :patients do |t|
t.string :name
t.timestamps
end
create_table :appointments do |t|
t.belongs_to :physician
t.belongs_to :patient
t.datetime :appointment_date
t.timestamps
end
end
end
```
The collection of join models can be managed via the API. For example, if you assign
```ruby
@ -199,6 +281,31 @@ end
![has_one :through Association Diagram](images/has_one_through.png)
The corresponding migration might look like this:
```ruby
class CreateAccountHistories < ActiveRecord::Migration
def change
create_table :suppliers do |t|
t.string :name
t.timestamps
end
create_table :accounts do |t|
t.belongs_to :supplier
t.string :account_number
t.timestamps
end
create_table :account_histories do |t|
t.belongs_to :account
t.integer :credit_rating
t.timestamps
end
end
end
```
### The `has_and_belongs_to_many` Association
A `has_and_belongs_to_many` association creates a direct many-to-many connection with another model, with no intervening model. For example, if your application includes assemblies and parts, with each assembly having many parts and each part appearing in many assemblies, you could declare the models this way:
@ -215,6 +322,29 @@ end
![has_and_belongs_to_many Association Diagram](images/habtm.png)
The corresponding migration might look like this:
```ruby
class CreateAssembliesAndParts < ActiveRecord::Migration
def change
create_table :assemblies do |t|
t.string :name
t.timestamps
end
create_table :parts do |t|
t.string :part_number
t.timestamps
end
create_table :assemblies_parts do |t|
t.belongs_to :assembly
t.belongs_to :part
end
end
end
```
### Choosing Between `belongs_to` and `has_one`
If you want to set up a one-to-one relationship between two models, you'll need to add `belongs_to` to one, and `has_one` to the other. How do you know which is which?