Rails using scope to select "main Image"

In the index of my app, I'm showing all projects. Each project has multiple attachments. Each attachment has an image and a main_image boolean. I'm trying to loop thru the attachments and select the one with the boolean main_image set to true, and make the selected attachment image the project thumbnail.

I created a scope in the attachment model, hoping that I can use it to select the correct attachment for the thumbnail, but I can't figure out how to do this.

I think I'm approaching this the wrong way. Can someone please recommend how to archive my goal?

I researched some posts here: [Loop within Loop in Rails Controller

Attachment.rb

class Attachment < ApplicationRecord
  belongs_to :project
  scope :main_image, lambda {where("main_image = 1")}

end

Project.rb

class Project < ApplicationRecord
    has_many :attachments, dependent: :destroy
    has_and_belongs_to_many :categories
    before_save :set_default_position

    scope :active, lambda {where(:active =>  true).order("position ASC")}

#array of picture attachments  
    def attachments_array=(array)
       array.each do |file|
           attachments.build(:attachment => file)
       end
    end 

  def set_default_position
    if self.position == nil
      self.position = 1
    end
  end

end

welcome/index.html

<div id="Container" class="mixContainer">
    <% @projects.each do |project| %>
        <div class="mix <% project.categories.each do |cat| %>category-<%= cat.id %> <% end %>project-<%= project.id %>" data-myorder="<%= project.position %>">    
            <div class="mixContent">
                <% project.attachments.main_image.each do |attachment| %>
                 <%= image_tag attachment.image.url(:thumb), class:"img-responsive" %>
                <% end %>
            </div>        
            <%= link_to(project) do %>
            <div class="mixContentOver">
                <div class="thumbTitle">
                    <h2><%= project.title %></h2>                       
                </div>
                <div class="thumbDescription">
                    <h4><%= project.description %></h4>                       
                </div>
            </div>
            <% end %>
        </div>
    <% end %>

    </div>

welcome_controller.rb

class WelcomeController < ApplicationController

    def index
        @projects = Project.active

    end


end 

error:

ActionView::Template::Error (PG::UndefinedFunction: ERROR:  operator does not exist: boolean = integer
2016-09-11T20:13:59.328785+00:00 app[web.1]: LINE 1: ..." WHERE "attachments"."project_id" = $1 AND (main_image = 1)
2016-09-11T20:13:59.328786+00:00 app[web.1]:                                                                    ^
2016-09-11T20:13:59.328786+00:00 app[web.1]: HINT:  No operator matches the given name and argument type(s). You might need to add explicit type casts.
2016-09-11T20:13:59.328787+00:00 app[web.1]: : SELECT "attachments".* FROM "attachments" WHERE "attachments"."project_id" = $1 AND (main_image = 1)):
2016-09-11T20:13:59.329627+00:00 app[web.1]:     49:     <% @projects.each do |project| %>
2016-09-11T20:13:59.329796+00:00 app[web.1]:     50:         <div class="mix <% project.categories.each do |cat| %>category-<%= cat.id %> <% end %>project-<%= project.id %>" data-myorder="<%= project.position %>">    
2016-09-11T20:13:59.329956+00:00 app[web.1]:     51:             <div class="mixContent">
2016-09-11T20:13:59.329993+00:00 app[web.1]:     52:                 <% project.attachments.main_image.each do |attachment| %>
2016-09-11T20:13:59.330124+00:00 app[web.1]:     53:                  <%= image_tag attachment.image.url(:thumb), class:"img-responsive" %>
2016-09-11T20:13:59.330293+00:00 app[web.1]:     54:                 <% end %>
2016-09-11T20:13:59.330328+00:00 app[web.1]:     55:             </div>        
  [1]: https://stackoverflow.com/questions/10903919/loop-within-loop-in-rails-controller