Rails%203%20In%20Action

Restricting read access
This message occurs because you haven’t defined a belongs_to association between
the Permission objects and the users they relate to. To set up this association, open
app/models/permission.rb and define it using a simple belongs_to:
belongs_to :user
That’s the first association you need to define, and when you run this feature again,
you get a second error that looks similar to the first:
And "user@ticketee.com" can view the "TextMate 2" project
unknown attribute: thing (ActiveRecord::UnknownAttributeError)
This code represents a polymorphic association, which as mentioned earlier, needs to
associate with many types of objects. A polymorphic association uses the thing_type
and thing_id fields to determine what object a Permission object relates to.
To define this association in your model, use this line:
belongs_to :thing, :polymorphic => true
Figure 8.1 illustrates how this association works.
When you assign an object to the thing polymorphic association, instead of just
saving thing_id as in a normal belongs_to, Rails also saves the thing_type field,
which is the string version of the object’s class, or thing.class.to_s. In this step of
your application, the thing_type field is set to Project because you’re assigning a
Project object to thing. Therefore, the new record in the table has both thing_type
and thing_id attributes set.
When Rails loads this object, it goes through the process shown in figure 8.2. Rails
knows this is a polymorphic association because you told it in the Permission model,
and it therefore uses the thing_id and thing_type fields to find the object. By knowing
thing_type, Rails can figure out what model the association is and then use that
Permission
thing = Project(id = 1)
thing.id thing.class.to_s
Permission(thing_id
= thing.id)
Permission(thing_type =
thing.class.to_s)
Permission(thing_id = 1, thing_type = Project)
Figure 8.1 Polymorphic saving
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