Ruby: What is it declare in a class?

Question

I got this sample code from datamapper http://datamapper.org/getting-started.html

class Post
  include DataMapper::Resource

  property :id,         Serial    # An auto-increment integer key
  property :title,      String    # A varchar type string, for short strings
  property :body,       Text      # A text block, for longer string data.
  property :created_at, DateTime  # A DateTime, for any date you might like.
end

Can anyone tell me that how "property" generate? Is it a function, variable, class variable or instance variable or a constant?

sometime i also saw this kind of code

class CarModel
   attribute :name
   attribute :hello
end

but no idea how does this generate

I think you may be meaning something other than "how xxx generate". property is a method, as is attribute. Both are being called/invoked within the class definition. What they "do" is a separate question. — Peter Alfvin
– Peter Alfvin, Commented Dec 4, 2013 at 1:45
so are they a method bening invoked every thing when class is initialized? — Kit Ho
– Kit Ho, Commented Dec 4, 2013 at 1:50
property :id, Serial is just call of function with 2 arguments. It changes class (adds property). It's meta-programming tip. Ruby has a lot of it. — user2895892
– user2895892, Commented Dec 4, 2013 at 1:51

mechanicalfish · Accepted Answer · 2013-12-04 01:50:58Z

1

It is a method that is included when you do:

include DataMapper::Resource

You can see its source code here if you're interested in digging in deeper.

It basically adds a property to the list of properties in your Post resource.

answered Dec 4, 2013 at 1:50

mechanicalfish

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4 Comments

Kit Ho Over a year ago

so property function will be executed when we initialize the class?

mechanicalfish Over a year ago

Yes, it is called when the class is initialised (when its source code is loaded). Also Ruby does not have functions, it is a method.

Peter Alfvin Over a year ago

@KitHo Don't confuse "when class is initialized" with "when an instance of the class is created". property and attribute are only called once per their occurrence in the code - when the class itself is defined/initialized.

Kit Ho Over a year ago

Is there any name for this kind of pattern , so that i can search more on google. I never find this pattern in java/c++ code before

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