I usually come across people asking what is the preferred way of laying out the properties of a model and, in fact, of an ActiveRecord model. Shall we put first the validations and then the associations? Or first the associations and then the validations? Or shall we put first the callbacks?

One can google for rails style guide. Will basically find only this:

• https://github.com/bbatsov/rails-style-guide#models

which is a good resource. But is not complete.

My Rails Model Style Guidelines

we are talking here about Rails 3

Here is the list of my styling guidelines and which I use whenever I am writing a Model.

This is the sequence of declarations:

1. attr_accessor, and attr_reader, attr_writer for virtual attributes that are not derived from database
2. nilify_blanks. I am using the nilify_blanks gem and here is where I put the corresponding directive
3. attr_accessible, in order to declare the mass assignable attributes
4. associations
   1. all belongs_to,
   2. all has_one,
   3. all has_many (there is a gotcha here, read Note 1 below)
5. before validation callbacks
6. validations
7. after validation callbacks
8. delegates
9. scopes
10. callbacks
    1. First the before,
    2. then the after,
    3. then the around
       and according to the sequence that they take place. Read Rails Guides ActiveRecord Callbacks (there is a gotcha here, see Note 1 below) and EXCEPT the (before|after)_validation callbacks, since I put them earlier in the list
       
11. public methods
    1. class methods
    2. instance methods
12. protected methods
    1. class methods
    2. instance methods
13. private methods
    1. class methods
    2. instance methods

Note 1: the [has_many | has_one | belongs_to] associations allow you to define :dependent => :destroy (though I do not use it on belongs_to….is not correct design … IMHO). This creates a before_destroy callback. The before_destroy callbacks are executed in the sequence that they are defined. So, If you want a before_destroy callback to take place before a before_destroy callback of a has_one | has_many that destroys the dependent objects, you will have to put the before_destroy callback before the corresponding has_one | has_many.

This is a list of some extra guidelines:

• Whenever I have attr_accessor (or attr_reader, or attr_writer or attr_accessible) I put the list of corresponding attributes in alphabetical order. So, in long lists it becomes easier for me to locate a specific attribute.

• I tend to leave a blank line between two different groups of declarations. For example, when the validations block ends, I leave a blank line before I start the after validation callbacks.

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I would be glad to hear about yours in order to improve mine too. (Comments are welcome on Google+)