This post is one in a series on how I achieved composite classes and multi-table inheritance. For background, see
Composite classes and multi-table inheritance. The last post in the series was
Create EventBase.
For proper attribution, I'll note that a whole lot of the code in this post came directly from or was inspired by
Multiple Table Inheritance with ActiveRecord.
In this post, I'm going to be taking a first cut at the ActsAsEvent module. If you don't know what modules are or how to create and use them, do some Googling.
Including Your Modules
One thing to remember is that you have to require your modules in order for Rails to find them correctly. In my config/initializers file, I include the line:
Dir[File.join(Rails.root, "lib", "*.rb")].each {|l| require l }
which causes all files in my lib directory ending in .rb to be required.
The Basic ActsAsEvent Module
In my lib directory, I created an acts_as_event.rb file for my ActsAsEvent module. I edited the file to create a basic module structure:
module ActsAsEvent
def acts_as_event
class_eval do
include InstanceMethods
extend ClassMethods
end
end
module InstanceMethods
end # InstanceMethods
module ClassMethods
end # ClassMethods
end
ActiveRecord::Base.extend ActsAsEvent
At this point, the module includes one method declaration (acts_as_event) and two module declarations (InstanceMethods and ClassMethods).
The
class_eval block (within
acts_as_event) is a metaprogramming construct that allows anything defined in the block to be included in another class as if it had been defined in that class. Within this block, I included the include InstanceMethod and extend ClassMethods lines which cause any class that calls acts_as_event to include and extend the instance and class methods (respectively).
To the acts_as_event method, I added two lines:
def acts_as_event
class_eval do
include InstanceMethods
extend ClassMethods
has_one :event_base, as: :eventable, :autosave => true, :dependent => :destroy
alias_method_chain :event_base, :build
end
end
The
has_one :event_base line creates an association between any class calling the
acts_as_event method and the EventBase class that I created in a
previous post. If you are unfamiliar with Rails associations, see
A Guide to ActiveRecord Associations.
The
alias_method_chain :event_base, :build line ensures that an EventBase record is created each time the composing object (in my case, Event or Trip) is created. This line calls an event_base_with_build instance method which I declared as:
module InstanceMethods
def event_base_with_build
event_base_without_build || build_event_base
end
end # InstanceMethods
Mix the Method Into the Trip Class
Finally, I call the
acts_as_event method (that I defined in the ActsAsEvent module) from within my Trip class:
class Trip < ActiveRecord::Base
# ==========================================================================================
# Mixins
# ==========================================================================================
acts_as_event
end
When I call
acts_as_events, it causes the
class_eval block to be called which in turn 'mixes in' everything within that block.
Testing the Mixed In Module
At this point, I should have a functioning mixed in module. So, I wrote a simple test in
spec/models/trip_spec.rb:
require 'spec_helper'
describe Trip do
describe "functions properly" do
it " - saves EventBase correctly" do
tcount_before = Trip.find(:all).count
ebcount_before = EventBase.find(:all).count
@t = Trip.create
tid = @t.id
@t.respond_to?("event_base").should be_true
@t.event_base.description = "test description"
@t.save.should be_true
@t = nil
@dbt = Trip.find(tid)
@dbt.event_base.description.should == "test description"
Trip.find(:all).count.should == tcount_before + 1
EventBase.find(:all).count.should == ebcount_before + 1
@dbt.destroy
Trip.find(:all).count.should == tcount_before
EventBase.find(:all).count.should == ebcount_before
end
end
end
Here's what's going on in the test:
- To begin, I record how many Trip and EventBase records there are in the database.
- Then, I create a new Trip record and save its ID to tid.
- I check that the new Trip responds to "event_base" - making sure the association is working correctly.
- Then, I assign a description to the EventBase associated with the Trip.
- And make sure it saves correctly. To be sure the save works, I set @t to nil and then retrieve the record from the database using tid.
- Next, I make sure the record counts make sense.
- Then, I destroy the record I just created.
- And, again, make sure the record counts make sense.
At the console, I ran:
$ rspec trip_spec.rb --format documentation
And I got:
Trip
functions properly
- saves EventBase correctly
Finished in 0.30754 seconds
1 example, 0 failures
Right on! It works.
Next Up: Mixin Validations
So far, this seems pretty good. But, there's a problem lurking in here. I want to be able to have EventBase validations so that my classes (Event and Trip) only save if my EventBase validations pass. So, let's say I add a validation to EventBase that ensures that there is a description, like this:
class EventBase < ActiveRecord::Base
# ==========================================================================================
# Validations
# ==========================================================================================
validates_presence_of :description
# ==========================================================================================
# Associations
# ==========================================================================================
belongs_to :recur_rule,
dependent: :destroy
belongs_to :eventable, polymorphic: true, dependent: :destroy
end
And, I update my test like this:
require 'spec_helper'
describe Trip do
describe "responds to" do
end
describe "functions properly" do
it " - saves EventBase correctly" do
tcount_before = Trip.find(:all).count
ebcount_before = EventBase.find(:all).count
@t = Trip.create
tid = @t.id
@t.respond_to?("event_base").should be_true
@t.save.should be_false
@t.event_base.description = "test description"
@t.save.should be_true
@dbt = Trip.find(@t.id)
@dbt.event_base.description.should == "test description"
Trip.find(:all).count.should == tcount_before + 1
EventBase.find(:all).count.should == ebcount_before + 1
@dbt.destroy
Trip.find(:all).count.should == tcount_before
EventBase.find(:all).count.should == ebcount_before
end
end
end
Note the line that says @t.save.should be_false. This says that my save should fail because event_base.description is nil. When I run:
$ rspec trip_spec.rb --format documentation
I get:
Trip
functions properly
- saves EventBase correctly (FAILED - 1)
Failures:
1) Trip functions properly - saves EventBase correctly
Failure/Error: @t.save.should be_false
expected: false value
got: true
# ./trip_spec.rb:25:in `block (3 levels) in <top (required)>'
Finished in 0.08088 seconds
1 example, 1 failure
Oh no! @t is saving even though the validation condition is not met. I'll fix this in the next post - Mixin Validations.
