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Add migration example to Association Basics [ci skip]

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1 parent 1b32c06 commit 312234c5788d6b97de445ff3b31960a9d605b954 @rmcastil rmcastil committed Dec 5, 2012
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  1. +130 −0 guides/source/association_basics.md
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130 guides/source/association_basics.md
@@ -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?

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