When to use a lambda in Ruby on Rails?

http://augustl.com/blog/2008/procs_blocks_and_anonymous_functions/ has a run-down of what blocks/procs/lambdas are, how you can use them, and how they compare to functions in other languages. It definitely answers your question.

Do be aware that the last section 'A note on lambdas' mentions a point that is only true in Ruby 1.8 and changed in 1.9 - Ruby: Proc.new { 'waffles' } vs. proc { 'waffles' }

I don't see where you make the distinction between Ruby on Rails and Ruby. If you're writing a Ruby on Rails application, you're writing Ruby code, so if it's useful in Ruby, it should be useful in Ruby on Rails.

Anyway, this article, Some Useful Closures in Ruby, should be helpful, as well as this: http://www.robertsosinski.com/2008/12/21/understanding-ruby-blocks-procs-and-lambdas/

It is a piece of code that allows you to pass around.

It is especially useful in named_scope, it allows to you do something like this:

named_scope :scoped_by_user, lambda {|user| {:conditions=>{:user_id=>user.id}}}

Say you have a Project model and you want to get all the projects for one particular user, you can do something like this:

Project.scoped_by_user(123)

Where I've seen Lambda used is in testing...

lambda do
    post :create, :user => @attr
end.should_not change(User, :count)

Lambda allows you to have that test at the end to make sure that the code executed in the lambda block doesn't change change the User count.

lambda is exceptionally useful in named_scope, so that you can pass parameters to named_scopes.

Many validator options accept a lambda or Proc if the work is too simple to warrant a named function. For example, :if and :unless.

validates :thing, presence: true, unless: ->(obj) { obj.something? }

:message will accept a Proc for custom validator messages.

validates :terms, acceptance: {
    message: ->(person) "%{person.name} must accept our terms!"
}

Generally, lambdas (and callbacks in general) are useful as a lightweight way to allow more customization than a string or number allows, but without the user having to write a full class.