This guide will give you a strong grasp of the Active Record pattern and how it can be used with or without Rails. Hopefully, some of the philosophical and theoretical intentions discussed here will also make you a stronger and better developer.
Rails' ActiveRecord is an implementation of Martin Fowler's http://martinfowler.com/eaaCatalog/activeRecord.html[Active Record Design Pattern]. This pattern is based on the idea of creating relations between the database and the application in the following way:
The relation between databases and object-oriented software is called ORM, which is short for "Object Relational Mapping". The purpose of an ORM framework is to minimize the mismatch existent between relational databases and object-oriented software. In applications with a domain model, we have objects that represent both the state of the system and the behaviour of the real world elements that were modeled through these objects. Since we need to store the system's state somehow, we can use relational databases, which are proven to be an excelent approach to data management. Usually this may become a very hard thing to do, since we need to create an object-oriented model of everything that lives in the database, from simple columns to complicated relations between different tables. Doing this kind of thing by hand is a tedious and error prone job. This is where an ORM framework comes in.
Active Record plays the role of model inside the MVC structure followed by Rails applications. Since model objects should encapsulate both state and logic of your applications, it's ActiveRecord responsability to deliver you the easiest possible way to recover this data from the database.
When writing applications using other programming languages or frameworks, it may be necessary to write a lot of configuration code. This is particulary 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 ActiveRecord 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, explicity configuration would be needed only in those cases where you can't follow the conventions for any reason.
By default, ActiveRecord 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 i.e. (book_clubs)
* Model Class - Singular with the first letter of each word capitalized i.e. (BookClub)
[width="60%", options="header"]
|==============================
|Model / Class |Table / Schema
|Post |posts
|LineItem |line_items
|Deer |deer
|Mouse |mice
|Person |people
|==============================
=== Schema Conventions
ActiveRecord 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 table_id i.e. (item_id, order_id). These are the fields that ActiveRecord will look for when you create associations between your models.
* *Primary keys* - By default, ActiveRecord will use a integer column named "id" as the table's primary key. When using http://guides.rails.info/migrations.html[Rails Migrations] to create your tables, this column will be automaticaly created.
There are also some optional column names that will create additional features to ActiveRecord instances:
* *created_at / created_on* - ActiveRecord will store the current date and time to this field when creating the record.
* *updated_at / updated_on* - ActiveRecord will store the current date and times to this field when updating the record.
* *lock_version* - Adds http://api.rubyonrails.com/classes/ActiveRecord/Locking.html[optimistic locking] to a model.
* *type* - Specifies that the model uses http://api.rubyonrails.com/classes/ActiveRecord/Base.html[Single Table Inheritance]
* *(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 ActiveRecord. 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. If you are not using STI, try an analogous keyword like "context", that may still accurately describe the data you are modeling.
== Creating ActiveRecord models
It's very easy to create ActiveRecord models. All you have to do is to subclass the ActiveRecord::Base class and you're good to go:
This will create a +Product+ model, mapped to a *products* table at the database. By doing this you'll also have the hability to map the columns of each row in that table with the attributes of the instances of your model. So, suppose that the *products* table was created using a SQL sentence like:
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.
It's also possible to override the column that should be used as the table's primary key. Use the +ActiveRecord::Base.set_primary_key+ method for that:
ActiveRecord gives the hability to validate the state of your models before they get recorded into the database. There are several methods that you can use to hook into the lifecycle of your models and validate that an attribute value is not empty or follow a specific format and so on. You can learn more about validations in the http://guides.rails.info/activerecord_validations_callbacks.html#_overview_of_activerecord_validation[Active Record Validations and Callbacks guide].
== Callbacks
ActiveRecord callbacks allow you to attach code to certain events in the lifecycle of your models. This way you can add behaviour 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 http://guides.rails.info/activerecord_validations_callbacks.html#_callbacks[Active Record Validations and Callbacks guide].