Skip to content

HTTPS clone URL

Subversion checkout URL

You can clone with HTTPS or Subversion.

Download ZIP
Browse files

Merge pull request #8496 from ptn/link-AR-basics-guide

Link to Active Record Basics guide [ci skip]
  • Loading branch information...
commit 0a6ee1f541b19be6bbbcdb7aa04418dd59da6810 2 parents fe70cde + dcf0b2d
@steveklabnik steveklabnik authored
Showing with 186 additions and 42 deletions.
  1. +182 −42 guides/source/active_record_basics.md
  2. +4 −0 guides/source/documents.yaml
View
224 guides/source/active_record_basics.md
@@ -5,9 +5,11 @@ This guide is an introduction to Active Record.
After reading this guide, you will know:
-* What Object Relational Mapping and Active Record are and how they are used in Rails.
+* What Object Relational Mapping and Active Record are and how they are used in
+ Rails.
* How Active Record fits into the Model-View-Controller paradigm.
-* How to use Active Record models to manipulate data stored in a relational database.
+* How to use Active Record models to manipulate data stored in a relational
+ database.
* Active Record schema naming conventions.
* The concepts of database migrations, validations and callbacks.
@@ -16,19 +18,34 @@ After reading this guide, you will know:
What is Active Record?
----------------------
-Active Record is the M in [MVC](getting_started.html#the-mvc-architecture) - the model - which is the layer of the system responsible for representing business data and logic. Active Record facilitates the creation and use of business objects whose data requires persistent storage to a database. It is an implementation of the Active Record pattern which itself is a description of an Object Relational Mapping system.
+Active Record is the M in [MVC](getting_started.html#the-mvc-architecture) - the
+model - which is the layer of the system responsible for representing business
+data and logic. Active Record facilitates the creation and use of business
+objects whose data requires persistent storage to a database. It is an
+implementation of the Active Record pattern which itself is a description of an
+Object Relational Mapping system.
### The Active Record Pattern
-Active Record was described by Martin Fowler in his book _Patterns of Enterprise Application Architecture_. In Active Record, objects carry both persistent data and behavior which operates on that data. Active Record takes the opinion that ensuring data access logic is part of the object will educate users of that object on how to write to and read from the database.
+Active Record was described by Martin Fowler in his book _Patterns of Enterprise
+Application Architecture_. In Active Record, objects carry both persistent data
+and behavior which operates on that data. Active Record takes the opinion that
+ensuring data access logic is part of the object will educate users of that
+object on how to write to and read from the database.
### Object Relational Mapping
-Object-Relational Mapping, commonly referred to as its abbreviation ORM, is a technique that connects the rich objects of an application to tables in a relational database management system. Using ORM, the properties and relationships of the objects in an application can be easily stored and retrieved from a database without writing SQL statements directly and with less overall database access code.
+Object-Relational Mapping, commonly referred to as its abbreviation ORM, is
+a technique that connects the rich objects of an application to tables in
+a relational database management system. Using ORM, the properties and
+relationships of the objects in an application can be easily stored and
+retrieved from a database without writing SQL statements directly and with less
+overall database access code.
### Active Record as an ORM Framework
-Active Record gives us several mechanisms, the most important being the ability to:
+Active Record gives us several mechanisms, the most important being the ability
+to:
* Represent models and their data
* Represent associations between these models
@@ -39,14 +56,30 @@ Active Record gives us several mechanisms, the most important being the ability
Convention over Configuration in Active Record
----------------------------------------------
-When writing applications using other programming languages or frameworks, it may be necessary to write a lot of configuration code. This is particularly true for ORM frameworks in general. However, if you follow the conventions adopted by Rails, you'll need to write very little configuration (in some case no configuration at all) when creating Active Record models. The idea is that if you configure your applications in the very same way most of the times then this should be the default way. In this cases, explicit configuration would be needed only in those cases where you can't follow the conventions for any reason.
+When writing applications using other programming languages or frameworks, it
+may be necessary to write a lot of configuration code. This is particularly true
+for ORM frameworks in general. However, if you follow the conventions adopted by
+Rails, you'll need to write very little configuration (in some case no
+configuration at all) when creating Active Record models. The idea is that if
+you configure your applications in the very same way most of the times then this
+should be the default way. In this cases, explicit configuration would be needed
+only in those cases where you can't follow the conventions for any reason.
### Naming Conventions
-By default, Active Record uses some naming conventions to find out how the mapping between models and database tables should be created. Rails will pluralize your class names to find the respective database table. So, for a class `Book`, you should have a database table called **books**. The Rails pluralization mechanisms are very powerful, being capable to pluralize (and singularize) both regular and irregular words. When using class names composed of two or more words, the model class name should follow the Ruby conventions, using the CamelCase form, while the table name must contain the words separated by underscores. Examples:
+By default, Active Record uses some naming conventions to find out how the
+mapping between models and database tables should be created. Rails will
+pluralize your class names to find the respective database table. So, for
+a class `Book`, you should have a database table called **books**. The Rails
+pluralization mechanisms are very powerful, being capable to pluralize (and
+singularize) both regular and irregular words. When using class names composed
+of two or more words, the model class name should follow the Ruby conventions,
+using the CamelCase form, while the table name must contain the words separated
+by underscores. Examples:
* Database Table - Plural with underscores separating words (e.g., `book_clubs`)
-* Model Class - Singular with the first letter of each word capitalized (e.g., `BookClub`)
+* Model Class - Singular with the first letter of each word capitalized (e.g.,
+`BookClub`)
| Model / Class | Table / Schema |
| ------------- | -------------- |
@@ -59,34 +92,52 @@ By default, Active Record uses some naming conventions to find out how the mappi
### Schema Conventions
-Active Record uses naming conventions for the columns in database tables, depending on the purpose of these columns.
-
-* **Foreign keys** - These fields should be named following the pattern `singularized_table_name_id` (e.g., `item_id`, `order_id`). These are the fields that Active Record will look for when you create associations between your models.
-* **Primary keys** - By default, Active Record will use an integer column named `id` as the table's primary key. When using [Rails Migrations](migrations.html) to create your tables, this column will be automatically created.
-
-There are also some optional column names that will create additional features to Active Record instances:
-
-* `created_at` - Automatically gets set to the current date and time when the record is first created.
-* `created_on` - Automatically gets set to the current date when the record is first created.
-* `updated_at` - Automatically gets set to the current date and time whenever the record is updated.
-* `updated_on` - Automatically gets set to the current date whenever the record is updated.
-* `lock_version` - Adds [optimistic locking](http://api.rubyonrails.org/classes/ActiveRecord/Locking.html) to a model.
-* `type` - Specifies that the model uses [Single Table Inheritance](http://api.rubyonrails.org/classes/ActiveRecord/Base.html)
-* `(table_name)_count` - Used to cache the number of belonging objects on associations. For example, a `comments_count` column in a `Post` class that has many instances of `Comment` will cache the number of existent comments for each post.
+Active Record uses naming conventions for the columns in database tables,
+depending on the purpose of these columns.
+
+* **Foreign keys** - These fields should be named following the pattern
+ `singularized_table_name_id` (e.g., `item_id`, `order_id`). These are the
+ fields that Active Record will look for when you create associations between
+ your models.
+* **Primary keys** - By default, Active Record will use an integer column named
+ `id` as the table's primary key. When using [Rails
+ Migrations](migrations.html) to create your tables, this column will be
+ automatically created.
+
+There are also some optional column names that will create additional features
+to Active Record instances:
+
+* `created_at` - Automatically gets set to the current date and time when the
+ record is first created.
+* `updated_at` - Automatically gets set to the current date and time whenever
+ the record is updated.
+* `lock_version` - Adds [optimistic
+ locking](http://api.rubyonrails.org/classes/ActiveRecord/Locking.html) to
+ a model.
+* `type` - Specifies that the model uses [Single Table
+ Inheritance](http://api.rubyonrails.org/classes/ActiveRecord/Base.html)
+* `(table_name)_count` - Used to cache the number of belonging objects on
+ associations. For example, a `comments_count` column in a `Post` class that
+ has many instances of `Comment` will cache the number of existent comments
+ for each post.
NOTE: While these column names are optional, they are in fact reserved by Active Record. Steer clear of reserved keywords unless you want the extra functionality. For example, `type` is a reserved keyword used to designate a table using Single Table Inheritance (STI). If you are not using STI, try an analogous keyword like "context", that may still accurately describe the data you are modeling.
Creating Active Record Models
-----------------------------
-It is very easy to create Active Record models. All you have to do is to subclass the `ActiveRecord::Base` class and you're good to go:
+It is very easy to create Active Record models. All you have to do is to
+subclass the `ActiveRecord::Base` class and you're good to go:
```ruby
class Product < ActiveRecord::Base
end
```
-This will create a `Product` model, mapped to a `products` table at the database. By doing this you'll also have the ability to map the columns of each row in that table with the attributes of the instances of your model. Suppose that the `products` table was created using an SQL sentence like:
+This will create a `Product` model, mapped to a `products` table at the
+database. By doing this you'll also have the ability to map the columns of each
+row in that table with the attributes of the instances of your model. Suppose
+that the `products` table was created using an SQL sentence like:
```sql
CREATE TABLE products (
@@ -96,7 +147,8 @@ CREATE TABLE products (
);
```
-Following the table schema above, you would be able to write code like the following:
+Following the table schema above, you would be able to write code like the
+following:
```ruby
p = Product.new
@@ -107,9 +159,12 @@ puts p.name # "Some Book"
Overriding the Naming Conventions
---------------------------------
-What if you need to follow a different naming convention or need to use your Rails application with a legacy database? No problem, you can easily override the default conventions.
+What if you need to follow a different naming convention or need to use your
+Rails application with a legacy database? No problem, you can easily override
+the default conventions.
-You can use the `ActiveRecord::Base.table_name=` method to specify the table name that should be used:
+You can use the `ActiveRecord::Base.table_name=` method to specify the table
+name that should be used:
```ruby
class Product < ActiveRecord::Base
@@ -117,7 +172,9 @@ class Product < ActiveRecord::Base
end
```
-If you do so, you will have to define manually the class name that is hosting the fixtures (class_name.yml) using the `set_fixture_class` method in your test definition:
+If you do so, you will have to define manually the class name that is hosting
+the fixtures (class_name.yml) using the `set_fixture_class` method in your test
+definition:
```ruby
class FunnyJoke < ActiveSupport::TestCase
@@ -127,7 +184,8 @@ class FunnyJoke < ActiveSupport::TestCase
end
```
-It's also possible to override the column that should be used as the table's primary key using the `ActiveRecord::Base.set_primary_key` method:
+It's also possible to override the column that should be used as the table's
+primary key using the `ActiveRecord::Base.set_primary_key` method:
```ruby
class Product < ActiveRecord::Base
@@ -138,19 +196,24 @@ end
CRUD: Reading and Writing Data
------------------------------
-CRUD is an acronym for the four verbs we use to operate on data: **C**reate, **R**ead, **U**pdate and **D**elete. Active Record automatically creates methods to allow an application to read and manipulate data stored within its tables.
+CRUD is an acronym for the four verbs we use to operate on data: **C**reate,
+**R**ead, **U**pdate and **D**elete. Active Record automatically creates methods
+to allow an application to read and manipulate data stored within its tables.
### Create
-Active Record objects can be created from a hash, a block or have their attributes manually set after creation. The `new` method will return a new object while `create` will return the object and save it to the database.
+Active Record objects can be created from a hash, a block or have their
+attributes manually set after creation. The `new` method will return a new
+object while `create` will return the object and save it to the database.
-For example, given a model `User` with attributes of `name` and `occupation`, the `create` method call will create and save a new record into the database:
+For example, given a model `User` with attributes of `name` and `occupation`,
+the `create` method call will create and save a new record into the database:
```ruby
user = User.create(name: "David", occupation: "Code Artist")
```
-Using the `new` method, an object can be created without being saved:
+Using the `new` method, an object can be instantiated without being saved:
```ruby
user = User.new
@@ -160,7 +223,8 @@ user.occupation = "Code Artist"
A call to `user.save` will commit the record to the database.
-Finally, if a block is provided, both `create` and `new` will yield the new object to that block for initialization:
+Finally, if a block is provided, both `create` and `new` will yield the new
+object to that block for initialization:
```ruby
user = User.new do |u|
@@ -171,7 +235,8 @@ end
### Read
-Active Record provides a rich API for accessing data within a database. Below are a few examples of different data access methods provided by Active Record.
+Active Record provides a rich API for accessing data within a database. Below
+are a few examples of different data access methods provided by Active Record.
```ruby
# return array with all records
@@ -193,11 +258,13 @@ david = User.find_by_name('David')
users = User.where(name: 'David', occupation: 'Code Artist').order('created_at DESC')
```
-You can learn more about querying an Active Record model in the [Active Record Query Interface](active_record_querying.html) guide.
+You can learn more about querying an Active Record model in the [Active Record
+Query Interface](active_record_querying.html) guide.
### Update
-Once an Active Record object has been retrieved, its attributes can be modified and it can be saved to the database.
+Once an Active Record object has been retrieved, its attributes can be modified
+and it can be saved to the database.
```ruby
user = User.find_by_name('David')
@@ -205,9 +272,26 @@ user.name = 'Dave'
user.save
```
+A shorthand for this is to use a hash mapping attribute names to the desired
+value, like so:
+
+```ruby
+user = User.find_by_name('David')
+user.update_attributes(name: 'Dave')
+```
+
+This is most useful when updating several attributes at once. If, on the other
+hand, you'd like to update several records in bulk, you may find the
+`update_all` class method useful:
+
+```ruby
+User.update_all "max_login_attempts = 3, must_change_password = 'true'"
+```
+
### Delete
-Likewise, once retrieved an Active Record object can be destroyed which removes it from the database.
+Likewise, once retrieved an Active Record object can be destroyed which removes
+it from the database.
```ruby
user = User.find_by_name('David')
@@ -217,14 +301,70 @@ user.destroy
Validations
-----------
-Active Record allows you to validate the state of a model before it gets written into the database. There are several methods that you can use to check your models and validate that an attribute value is not empty, is unique and not already in the database, follows a specific format and many more. You can learn more about validations in the [Active Record Validations and Callbacks guide](active_record_validations_callbacks.html#validations-overview).
+Active Record allows you to validate the state of a model before it gets written
+into the database. There are several methods that you can use to check your
+models and validate that an attribute value is not empty, is unique and not
+already in the database, follows a specific format and many more.
+
+Validation is a very important issue to consider when persisting to database, so
+the methods `create`, `save` and `update_attributes` take it into account when
+running: they return `false` when validation fails and they didn't actually
+perform any operation on database. All of these have a bang counterpart (that
+is, `create!`, `save!` and `update_attributes!`), which are stricter in that
+they raise the exception `ActiveRecord::RecordInvalid` if validation fails.
+A quick example to illustrate:
+
+```ruby
+class User < ActiveRecord::Base
+ validates_presence_of :name
+end
+
+User.create # => false
+User.create! # => ActiveRecord::RecordInvalid: Validation failed: Name can't be blank
+```
+
+You can learn more about validations in the [Active Record Validations
+guide](active_record_validations.html).
Callbacks
---------
-Active Record callbacks allow you to attach code to certain events in the life-cycle of your models. This enables you to add behavior to your models by transparently executing code when those events occur, like when you create a new record, update it, destroy it and so on. You can learn more about callbacks in the [Active Record Validations and Callbacks guide](active_record_validations_callbacks.html#callbacks-overview).
+Active Record callbacks allow you to attach code to certain events in the
+life-cycle of your models. This enables you to add behavior to your models by
+transparently executing code when those events occur, like when you create a new
+record, update it, destroy it and so on. You can learn more about callbacks in
+the [Active Record Callbacks guide](active_record_callbacks.html).
Migrations
----------
-Rails provides a domain-specific language for managing a database schema called migrations. Migrations are stored in files which are executed against any database that Active Record support using rake. Rails keeps track of which files have been committed to the database and provides rollback features. You can learn more about migrations in the [Active Record Migrations guide](migrations.html)
+Rails provides a domain-specific language for managing a database schema called
+migrations. Migrations are stored in files which are executed against any
+database that Active Record support using `rake`. Here's a migration that
+creates a table:
+
+```ruby
+class CreatePublications < ActiveRecord::Migration
+ def change
+ create_table :publications do |t|
+ t.string :title
+ t.text :description
+ t.references :publication_type
+ t.integer :publisher_id
+ t.string :publisher_type
+ t.boolean :single_issue
+
+ t.timestamps
+ end
+ add_index :publications, :publication_type_id
+ end
+end
+```
+
+Rails keeps track of which files have been committed to the database and
+provides rollback features. To actually create the table, you'd run `rake db:migrate`
+and to roll it back, `rake db:rollback`.
+
+Note that the above code is database-agnostic: it will run in MySQL, postgresql,
+Oracle and others. You can learn more about migrations in the [Active Record
+Migrations guide](migrations.html)
View
4 guides/source/documents.yaml
@@ -9,6 +9,10 @@
name: Models
documents:
-
+ name: Active Record Basics
+ url: active_record_basics.html
+ description: This guide will get you started with models, persistence to database and the Active Record pattern and library.
+ -
name: Rails Database Migrations
url: migrations.html
description: This guide covers how you can use Active Record migrations to alter your database in a structured and organized manner.
Please sign in to comment.
Something went wrong with that request. Please try again.