Never, unless you have specific need for it. Automatic database-backed accessors are created for you, so you don't need to worry.
Any attr_accessors you do create will change the relevant @attr on the rails object, but this will be lost when the object is destroyed, unless you stick it back in the database. Sometimes you do want this behavior, but it's unusual in a rails app.
Now in ruby, it's a different story, and you end up using these very frequently. But I'd be surprised if you need them in rails---especially initially.
Let's say you have a class Person.
class Person
end
person = Person.new
person.name # => no method error
Obviously we never defined method name. Let's do that.
class Person
def name
@name # simply returning an instance variable @name
end
end
person = Person.new
person.name # => nil
person.name = "Dennis" # => no method error
Aha, we can read the name, but that doesn't mean we can assign the name. Those are two different methods. The former is called reader and latter is called writer. We didn't create the writer yet so let's do that.
class Person
def name
@name
end
def name=(str)
@name = str
end
end
person = Person.new
person.name = 'Dennis'
person.name # => "Dennis"
Awesome. Now we can write and read instance variable @name using reader and writer methods. Except, this is done so frequently, why waste time writing these methods every time? We can do it easier.
class Person
attr_reader :name
attr_writer :name
end
Even this can get repetitive. When you want both reader and writer just use accessor!
class Person
attr_accessor :name
end
person = Person.new
person.name = "Dennis"
person.name # => "Dennis"
Works the same way! And guess what: the instance variable @name in our person object will be set just like when we did it manually, so you can use it in other methods.
class Person
attr_accessor :name
def greeting
"Hello #{@name}"
end
end
person = Person.new
person.name = "Dennis"
person.greeting # => "Hello Dennis"
That's it. In order to understand how attr_reader, attr_writer, and attr_accessor methods actually generate methods for you, read other answers, books, ruby docs.
Best Answer
attr_accessoris a core feature of Ruby and is used to generate instance variables with getter and setter methods. Its use is never required in basic Ruby (it's a convenience).In the case of ActiveRecord models, getters and setters are already generated by ActiveRecord for your data columns.
attr_accessoris not needed or desirable.If you have additional instance data you don't need to persist (i.e. it's not a database column), you could then use
attr_accessorto save yourself a few lines of code.The similarly-named
attr_accessible— which is frequently seen in Rails code and confused withattr_accessor— is a deprecated method of controlling mass assignment within ActiveRecord models. Rails 4 doesn't support it out of the box; it has been replaced by Strong Parameters, which allows more granular control.