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Simple Example of Dependency Injection with Rails

Today I came across a great opportunity to illustrate dependency injection in a simple context. I had a Rails partial that was duplicated across two subclasses. The partial was responsible for displaying options to create a new record from the data of the current record. It also offered two types of copy, shallow and deep. The shallow copy used a button to POST data, while the deep copy offered a form with some additional options. The only difference between the partials was the path to post data to. Let's see this in code.

#app/views/fun_event/_copy_options.html.erb
button_to(t("create_and_edit_shallow_copy"), fun_event_path(:from_event => @event.id, :return => true), :    id => "shallow_copy_btn")

form_tag(fun_event_path(:return => true)) do
  #form code
end


#app/views/boring_event/_copy_options.html.erb
button_to(t("create_and_edit_shallow_copy"), boring_event_path(:from_event => @event.id, :return => true), :    id => "shallow_copy_btn")

form_tag(boring_event_path(:return => true)) do
  #form code
end

The first, failed iteration

To remove the duplication, I passed in a path option into the partial, replacing specific references with the generic.

#app/views/fun_events/copy.html.erb
<%= render :partial => "events/copy_options", :event_path => fun_event_path %>


#app/views/boring_events/copy.html.erb
<%= render :partial => "events/copy_options, :event_path => boring_event_path %>

#app/views/events/_copy_options.html.erb
button_to(t("create_and_edit_shallow_copy"), event_path(:from_event => @event.id, :return => true), :    id => "shallow_copy_btn")

form_tag(event_path(:return => true)) do
  #form code
end

Can you guess where this led?

undefined method `event_path' for ActionView::Base:0xd6acf18

Dude! Inject the dependency!

Obviously the event_path variable I was passing was a string, not a method. I needed the method so I could pass in the appropriate arguments to construct the URL I needed. Had there not been two different calls to the routes, I would likely have just passed in the string needed in each context. But in this case, I was forced to think outside the box. Here's what I ended up with.

#app/views/fun_events/copy.html.erb
<%= render :partial => "events/copy_options", :event_path => method(:fun_event_path) %>


#app/views/boring_events/copy.html.erb
<%= render :partial => "events/copy_options, :event_path => method(:boring_event_path) %>

#app/views/events/_copy_options.html.erb
button_to(t("create_and_edit_shallow_copy"), event_path.call(:from_event => @event.id, :return => true), :    id => "shallow_copy_btn")

form_tag(event_path.call(:return => true)) do
  #form code
end

The changes are really quite subtle, but we use Object's method method to pass the reference to the method we want to call, and simply pass in the arguments when needed. Mind == Blown

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