Fix "missing attribute: state" when state attribute is not included in the query

[smartinm]

When limiting the fields returned from the database the error is raised:

[1] pry(main)> i = Model.all.only(:_id).first
ActiveModel::MissingAttributeError: Missing attribute: 'state'.
from /Users/sergio/.rvm/gems/ruby-2.2.0/gems/mongoid-4.0.2/lib/mongoid/attributes.rb:97:in `read_attribute'

This PR excludes state initialization when the attribute is not included in the query.

[look]

I just ran into this problem when upgrading an application from Mongoid 3 to Mongoid 5. This patch fixes the issue. It would be great to have it merged and push out a new release.

[scebotari]

Had the same issue when upgrading my application. Is this PR going to be merged?

@smartinm Thanks for this fix. Worked for my case too.

[sos4nt]

Since the problem still exists and this PR doesn't seem to get merged, here's a workaround:

The error is (at least in my case) caused by the state's initial value because the corresponding attribute is missing from the document when using only. This can be circumvented by setting the initial value in a lambda.

So instead of:

state_machine :state, initial: :parked do
  # ...
end

You can write:

state_machine :state, initial: -> { :parked } do
  state :parked # <- only needed when there's no transition to or from :parked

  # ...
end

This defers setting the initial value until you actually attempt to interact with the state machine.