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diff --git a/guides/source/active_record_basics.md b/guides/source/active_record_basics.md index c90f42c492..34baae509b 100644 --- a/guides/source/active_record_basics.md +++ b/guides/source/active_record_basics.md @@ -5,10 +5,10 @@ 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. @@ -18,107 +18,110 @@ 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 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 -* 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` | +| `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 +* `type` - Specifies that the model uses [Single Table + Inheritance](http://api.rubyonrails.org/classes/ActiveRecord/Base.html). +* `(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 `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 +129,7 @@ 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 +137,9 @@ 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 +150,7 @@ 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 +162,11 @@ 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 +175,41 @@ 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 +226,7 @@ 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 +238,22 @@ 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 +261,30 @@ 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 = 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 +293,57 @@ 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` 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 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 ``` -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 @@ -361,10 +364,10 @@ 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 -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). |