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diff --git a/guides/source/active_record_basics.md b/guides/source/active_record_basics.md index 883c2dda4a..9d2ba328ea 100644 --- a/guides/source/active_record_basics.md +++ b/guides/source/active_record_basics.md @@ -1,3 +1,5 @@ +**DO NOT READ THIS FILE IN GITHUB, GUIDES ARE PUBLISHED IN http://guides.rubyonrails.org.** + Active Record Basics ==================== @@ -5,10 +7,10 @@ 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. @@ -18,115 +20,118 @@ 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](http://www.martinfowler.com/eaaCatalog/activeRecord.html) +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 as 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 -* Represent associations between these models -* Represent inheritance hierarchies through related models -* Validate models before they get persisted to the database +* Represent models and their data. +* Represent associations between these models. +* Represent inheritance hierarchies through related models. +* Validate models before they get persisted to the database. * Perform database operations in an object-oriented fashion. 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 time then this +should be the default way. Thus, explicit configuration would be needed +only in those cases where you can't follow the standard convention. ### 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., -`BookClub`) +* 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 | Table / Schema | -| ------------- | -------------- | -| `Post` | `posts` | -| `LineItem` | `line_items` | -| `Deer` | `deer` | -| `Mouse` | `mice` | -| `Person` | `people` | +| Model / Class | Table / Schema | +| ---------------- | -------------- | +| `Article` | `articles` | +| `LineItem` | `line_items` | +| `Deer` | `deers` | +| `Mouse` | `mice` | +| `Person` | `people` | ### 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 [Active Record + 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 add 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 - 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. +* `type` - Specifies that the model uses [Single Table + Inheritance](http://api.rubyonrails.org/classes/ActiveRecord/Base.html#class-ActiveRecord::Base-label-Single+table+inheritance). +* `(association_name)_type` - Stores the type for + [polymorphic associations](association_basics.html#polymorphic-associations). +* `(table_name)_count` - Used to cache the number of belonging objects on + associations. For example, a `comments_count` column in a `Articles` class that + has many instances of `Comment` will cache the number of existent comments + for each article. 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 +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 +139,9 @@ 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 +152,7 @@ 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 +164,11 @@ 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 @@ -172,41 +177,41 @@ 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 class FunnyJoke < ActiveSupport::TestCase - set_fixture_class funny_jokes: 'Joke' + set_fixture_class funny_jokes: Joke fixtures :funny_jokes ... 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.primary_key=` method: ```ruby class Product < ActiveRecord::Base - set_primary_key "product_id" + self.primary_key = "product_id" 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 +228,7 @@ 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 @@ -235,22 +240,22 @@ 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 -# return array with all records +# return a collection with all users users = User.all ``` ```ruby -# return the first record +# return the first user user = User.first ``` ```ruby # return the first user named David -david = User.find_by_name('David') +david = User.find_by(name: 'David') ``` ```ruby @@ -258,30 +263,30 @@ 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 -user = User.find_by_name('David') +user = User.find_by(name: 'David') 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 -user = User.find_by_name('David') -user.update_attributes(name: 'Dave') +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 @@ -290,57 +295,58 @@ 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 -user = User.find_by_name('David') +user = User.find_by(name: 'David') 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_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. +Validation is a very important issue to consider when persisting to the database, so +the methods `save` and `update` take it into account when +running: they return `false` when validation fails and they didn't actually +perform any operation on the database. All of these have a bang counterpart (that +is, `save!` and `update!`), 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 + validates :name, presence: true end -User.create # => false -User.create! # => ActiveRecord::RecordInvalid: Validation failed: Name can't be blank +user = User.new +user.save # => false +user.save! # => 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 supports using `rake`. Here's a migration that creates a table: ```ruby @@ -354,17 +360,17 @@ class CreatePublications < ActiveRecord::Migration t.string :publisher_type t.boolean :single_issue - t.timestamps + t.timestamps null: false end add_index :publications, :publication_type_id end 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 -Migrations guide](migrations.html) +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). |