Introduce the Proposition model

Introduce an abstraction that is going to be used to propose models for
other models that can accept propositions.

app/models/proposition.rb

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+class Proposition < ActiveRecord::Base
+  belongs_to :proposer, class_name: 'User'
+  belongs_to :proposition_accepting, polymorphic: true
+  belongs_to :proposable, polymorphic: true
+end

db/migrate/20141105100105_create_propositions.rb

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+class CreatePropositions < ActiveRecord::Migration
+  def change
+    create_table :propositions do |t|
+      t.references :proposer, index: true
+      t.references :proposition_accepting, polymorphic: true
+      t.index [:proposition_accepting_id, :proposition_accepting_type], name: 'proposition_accepting_index'
+      t.references :proposable, polymorphic: true, index: true
+      t.integer :status
+
+      t.timestamps
+    end
+  end
+end

db/migrate/20141105100827_create_propositions_for_existing_events.rb

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+class CreatePropositionsForExistingEvents < ActiveRecord::Migration
+  def up
+    events = execute 'SELECT * FROM events'
+
+    events.each do |event|
+      execute "INSERT INTO propositions (proposer_id, proposition_accepting_id, proposition_accepting_type, proposable_id, proposable_type, status, created_at, updated_at)
+                 VALUES (#{event['user_id']}, #{event['track_id']}, 'Track', #{event['id']}, 'Event', #{event['state']}, '#{event['created_at']}', '#{event['updated_at']}')"
+    end
+  end
+
+  def down
+    event_ids = execute('SELECT * FROM events').map { |event| event['id'] }
+    execute "DELETE FROM propositions WHERE proposable_id IN (#{event_ids.join(', ')}) AND proposable_type = 'Event'"
+  end
+end

spec/factories/events.rb

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+FactoryGirl.define do
+  factory :event do
+    title { |n| "Event #{n}" }
+    length { 60 }
+    abstract { 'foo' }
+    description { 'foo' }
+  end
+end

spec/factories/propositions.rb

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+FactoryGirl.define do
+  factory :proposition do
+    association :proposer, factory: :user
+    association :proposition_accepting, factory: :track
+    association :proposable, factory: :event
+  end
+end

spec/models/proposition_spec.rb

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+require 'rails_helper'
+
+RSpec.describe Proposition, type: :model do
+  it 'belongs to a proposer' do
+    user = create :user
+    expect(create(:proposition, proposer: user).proposer).to eq user
+  end
+
+  it 'belongs to a proposition_accepting' do
+    track = create :track
+    expect(create(:proposition, proposition_accepting: track).proposition_accepting).to eq track
+  end
+
+  it 'belongs to a proposable' do
+    event = create :event
+    expect(create(:proposition, proposable: event).proposable).to eq event
+  end
+
+  it 'is invalid without an existing proposition_accepting'
+  it 'is invalid without an existing proposable'
+end