Chapter 3. Active Record Relationships


Dealing with relationships is one of the most important jobs of persistence frameworks. The best persistence frameworks handle relationships with excellent performance for the end user and simplicity for the developer. Active Record takes advantage of the Ruby language and naming conventions to simplify both access and configuration of related data. In this chapter, we'll focus on building relationships between tables, and reflecting those relationships in your model objects.

With validation, shown in the previous chapter, you began to see the domain-specific language built into Active Record. We'll use that language to define relationships between the objects in our database. Three components specify a relationship: the relationship itself, the association or target, and named parameters. More precisely, these are:



relationship

A method, defined through ActiveRecord::Base , which defines the behavior of the relationship.



association(s)

A symbol that specifies the target of the relationship. The symbol may be singular or plural, based on the cardinality of the target.



named parameters

Like all Ruby methods , the relationship can take an optional number of named parameters, which may also have default values.

A statement defining a relationship has the form:

 relationship :association :parameter1 => value, :parameter2 => value,... 

For example, you might have:

 class Slideshow < ActiveRecord::Base       has_many :photos :order => position 

Using this small amount of language, you'll be able to define complex relationships quickly. Your relationships will also be easy to read and maintain. Let's implement the full model for Photo Share, complete with relationships.

Relational Database Relationships

Relational databases are fundamentally based on different kinds of relationships between tables. A set of table columns called keys provides the structure for all relationships. A primary key is a set of columns in a table that uniquely identify a row within that same table. A foreign key is a set of columns in a table that uniquely identifies a row in another table. A database manager can join two tables by matching the primary keys in one table to the foreign keys in another. Active Record also uses primary and foreign keys to manage relationships. Unlike relational databases, Active Record limits its identifiers to a single database column.




Ruby on Rails[c] Up and Running
Ruby on Rails[c] Up and Running
ISBN: B003D3OGCY
EAN: N/A
Year: 2006
Pages: 94

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