Instance variables are variables that are local to a class. They’re preceded by an @ symbol. So if you have something like this:
class Shape
@name = "Unnamed"
end
@name and @sides is an instance variable. (Any fields inside the Shapes table are also instance variables.) You can do things like this:
triangle = Shape.new(:name => "Triangle", :sides => 3)
puts triangle.name # => "Triangle"
puts triangle.sides # => 3
In Rails, you can use instance variables to pass data between the controller and the view. You can write code like this:
# in shapes_controller.rb
class ShapesController < ApplicationController
def index
@shapes = Shape.find(:all)
end
end
# in views/shapes/index.html.erb
Listing Shapes
<% for shape in @shapes %>
<%=h shape.name %> (<%= h shape.sides %> sides)
<% end %>
This will list all of the shapes available in the application; you might see something like this:
Triangle (3 sides)
Circle (128 sides)
Square (4 sides)
Notice in our controller code, we assigned values from our model data to @shapes; we could then use that variable (technically, an instance variable in our ShapesController class) to access that data and display it in our view.
But how does it work? Isn’t @shapes an instance variable in the controller? It is–but Rails allows the view code to see it. In this way, Rails ties together the model, view, and controller aspects through instance variables in your controller classes.
And incidentally, this is the convention that the generated scaffolding code uses. Create any scaffolding, and it uses this same paradigm for all the default views (index, new, edit, show).
