Why is my has_one database relationship not working in rails?

Question

I have a relationship model in which students and employers can enter into a relationship when applying to a project. Here are the models:

class Student < User
    has_many :relationships, dependent: :destroy
    has_many :employers, through: :relationships
end

class Employer < User
    has_many :projects
    has_many :relationships, dependent: :destroy
    has_many :students, through: :relationships
end

class Relationship < ActiveRecord::Base
    has_one :project
    belongs_to :employer
    belongs_to :student

    validates_uniqueness_of :project_id, :scope => [:employer_id, :student_id]
end

class Project < ActiveRecord::Base
    belongs_to :employer
end

State is a column in the projects table that is automatically set equal to 'posting'. I am trying to show all relationships that have a project with state == :posting. Here is my view code:

<% @relationships.each do |relationship| %>
        <% if relationship.project.state == :posting %>
            <%= relationship.project.inspect %>
        <% end %>
<% end %>

Also, in the controller for this view is:

@relationships = Relationship.all

When i try to open the view, i get:

PG::UndefinedColumn: ERROR:  column projects.relationship_id does not exist

What doesn't make sense to me is that I am not looking in the projects table for a relationship_id column. My code should find the project from the relationship and then find the state column. I have relationships has_one :project but i don't have :projects belongs_to relationships because a project does not need a relationship in order to exist. I'm pretty sure my database relations are right. How do I fix my view code? Or, if my db relationships are wrong, what's wrong?

Update i forgot to mention that Employer and Student are both User models through polymorphic association. The user model has a type column that can be either Student or Employer.

Answer 1

When you declare that Relationship has_one :project, you are telling Rails that the Project model (and the corresponding projects database table) has a relationship_id field . When you refer to relationship.project in your view, you cause that table/field to be referenced, resulting in your error.

Given that each relationship should indeed have just one project associated with it, you should declare Project as belongs_to :relationship, create the appropriate migration and update your code accordingly to maintain the value of this field.

Answer 2

At first glance it seems like it should be

class Employer < User
  has_many :projects, through: :relationships

for this to work. Am I missing something?