Add missing files

app/models/category.rb

@@ -0,0 +1,4 @@
+class Category < ApplicationRecord
+  # rails already devine transaction method
+  has_one :transaction_, foreign_key: :category_id 
+end

db/migrate/20180930115502_create_categories.rb

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+class CreateCategories < ActiveRecord::Migration[5.2]
+  def change
+    create_table :categories do |t|
+      t.string :name 
+      t.string :color
+      t.string :help
+
+      t.timestamps
+    end
+  end
+end

db/migrate/20180930121500_add_category_to_transaction.rb

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+class AddCategoryToTransaction < ActiveRecord::Migration[5.2]
+  def change
+    add_reference :transactions, :category, foreign_key: true
+  end
+end

db/migrate/20180930123515_add_display_name_to_category.rb

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+class AddDisplayNameToCategory < ActiveRecord::Migration[5.2]
+  def change
+    add_column :categories, :display_name, :string
+    add_index :categories, %i[name], unique: true
+  end
+end

test/fixtures/categories.yml

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+# Read about fixtures at http://api.rubyonrails.org/classes/ActiveRecord/FixtureSet.html
+
+one:
+  name: MyString
+  color: MyString
+
+two:
+  name: MyString
+  color: MyString

test/models/category_test.rb

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+require 'test_helper'
+
+class CategoryTest < ActiveSupport::TestCase
+  # test "the truth" do
+  #   assert true
+  # end
+end