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@ -1,14 +1,14 @@
Active Record Basics
====================
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
* 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
* 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.
@ -18,33 +18,33 @@ 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
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
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
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
Active Record gives us several mechanisms, the most important being the ability
to:
* Represent models and their data
@ -56,29 +56,29 @@ to:
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
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 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.,
* Model Class - Singular with the first letter of each word capitalized (e.g.,
`BookClub`)
| Model / Class | Table / Schema |
@ -92,33 +92,33 @@ by underscores. Examples:
### Schema Conventions
Active Record uses naming conventions for the columns in database tables,
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
* **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
* **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
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
* `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
* `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
* `lock_version` - Adds [optimistic
locking](http://api.rubyonrails.org/classes/ActiveRecord/Locking.html) to
a model.
* `type` - Specifies that the model uses [Single Table
* `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
* `(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.
@ -126,7 +126,7 @@ NOTE: While these column names are optional, they are in fact reserved by Active
Creating Active Record Models
-----------------------------
It is very easy to create Active Record models. All you have to do is to
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
@ -134,9 +134,9 @@ 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
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
@ -147,7 +147,7 @@ CREATE TABLE products (
);
```
Following the table schema above, you would be able to write code like the
Following the table schema above, you would be able to write code like the
following:
```ruby
@ -159,11 +159,11 @@ 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
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
You can use the `ActiveRecord::Base.table_name=` method to specify the table
name that should be used:
```ruby
@ -172,8 +172,8 @@ 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
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
@ -184,7 +184,7 @@ class FunnyJoke < ActiveSupport::TestCase
end
```
It's also possible to override the column that should be used as the table's
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
@ -196,17 +196,17 @@ 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
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
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`,
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
@ -223,7 +223,7 @@ 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
Finally, if a block is provided, both `create` and `new` will yield the new
object to that block for initialization:
```ruby
@ -235,7 +235,7 @@ end
### Read
Active Record provides a rich API for accessing data within a database. Below
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
@ -258,12 +258,12 @@ 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
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
Once an Active Record object has been retrieved, its attributes can be modified
and it can be saved to the database.
```ruby
@ -272,7 +272,7 @@ user.name = 'Dave'
user.save
```
A shorthand for this is to use a hash mapping attribute names to the desired
A shorthand for this is to use a hash mapping attribute names to the desired
value, like so:
```ruby
@ -280,8 +280,8 @@ user = User.find_by_name('David')
user.update(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
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
@ -290,7 +290,7 @@ User.update_all "max_login_attempts = 3, must_change_password = 'true'"
### Delete
Likewise, once retrieved an Active Record object can be destroyed which removes
Likewise, once retrieved an Active Record object can be destroyed which removes
it from the database.
```ruby
@ -301,17 +301,17 @@ 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
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` 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!`), which are stricter in that
they raise the exception `ActiveRecord::RecordInvalid` if validation fails.
Validation is a very important issue to consider when persisting to database, so
the methods `create`, `save` and `update` 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!`), which are stricter in that
they raise the exception `ActiveRecord::RecordInvalid` if validation fails.
A quick example to illustrate:
```ruby
@ -323,24 +323,24 @@ 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
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
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`. Here's a migration that
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
@ -361,10 +361,10 @@ class CreatePublications < ActiveRecord::Migration
end
```
Rails keeps track of which files have been committed to the database and
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
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)