Call a block on a Model - ruby-on-rails

Question

Instead of defining a scope in a class like this:
scope :first_user, -> { first }
And calling it like this: User.first_user
I would like to define a block in another class, that can be called on the user class and works like a Scope:
This code is not working but it should signalize what behaviour I want to achieve:
class Manage
def get_first_user
User.&first_added
end
def first_added
Proc.new { first }
end
end
When I run this code:
a = Manage.new
a.get_first_user
it says me, & undefined method for User. How can I execute the Block defined in first_added on the User model?
How can I in general call a block on a class? Thanks

Answer 1

If I understand your question correctly, you can use class_eval:
>> foo = Proc.new { count }
=> #<Proc:0x007f1aa7cacfd8#(pry):30>
>> Buyer.class_eval(&foo)
(30.6ms) SELECT COUNT(*) FROM "buyers"
=> 1234
Or with your example:
class Manage
def get_first_user
User.class_eval(&first_added)
end
def first_added
Proc.new { first }
end
end

Answer 2

It is not what you want but maybe this will be helpful.
I am not sure if one can call proc on something. I think one can only call proc with something, i.t. in your case passing User as parameter.
def get_first_user
wrap User, &first_added
end
def first_added
Proc.new { |model| model.where(...) }
end
private
def wrap(model, &block)
yield model
end

Answer 3

Here's three ways to call first on User from Manager using:
Object#send:
http://ruby-doc.org/core-2.3.1/Object.html#method-i-send
Module#class_eval:
http://ruby-doc.org/core-2.3.1/Module.html#method-i-class_eval
File manage.rb:
class User
def self.first
puts 'record would probably go here'
end
def self.yielder
print "via yielder => "
self.send(yield) if block_given?
end
def self.blocker(&block)
print "via blocker => "
self.send(block.call) if block_given?
end
def self.evaller(&block)
print "via evaller => "
self.class_eval(block.call) if block_given?
end
end
class Manage
def get_first_user
User.yielder(&first_added)
User.blocker(&first_added)
User.evaller(&first_added)
end
def first_added
Proc.new {"first"}
end
end
a = Manage.new
a.get_first_user
Output:
$ ruby manage.rb
via yielder => record would probably go here
via blocker => record would probably go here
via evaller => record would probably go here