Added cop to blacklist the use of hash indexes

These indexes are not recorded in the WAL (at least until PostgreSQL
10) and this isn't worth the minor performance improvement over btree
indexes.

rubocop/cop/migration/hash_index.rb

+require 'set'
+require_relative '../../migration_helpers'
+
+module RuboCop
+  module Cop
+    module Migration
+      # Cop that prevents the use of hash indexes in database migrations
+      class HashIndex < RuboCop::Cop::Cop
+        include MigrationHelpers
+
+        MSG = 'hash indexes should be avoided at all costs since they are not ' \
+          'recorded in the PostgreSQL WAL, you should use a btree index instead'.freeze
+
+        NAMES = Set.new([:add_index, :index, :add_concurrent_index]).freeze
+
+        def on_send(node)
+          return unless in_migration?(node)
+
+          name = node.children[1]
+
+          return unless NAMES.include?(name)
+
+          opts = node.children.last
+
+          return unless opts && opts.type == :hash
+
+          opts.each_node(:pair) do |pair|
+            next unless hash_key_type(pair) == :sym &&
+                hash_key_name(pair) == :using
+
+            if hash_key_value(pair).to_s == 'hash'
+              add_offense(pair, :expression)
+            end
+          end
+        end
+
+        def hash_key_type(pair)
+          pair.children[0].type
+        end
+
+        def hash_key_name(pair)
+          pair.children[0].children[0]
+        end
+
+        def hash_key_value(pair)
+          pair.children[1].children[0]
+        end
+      end
+    end
+  end
+end

rubocop/rubocop.rb

@@ -13,6 +13,7 @@ require_relative 'cop/migration/add_concurrent_index'
 require_relative 'cop/migration/add_index'
 require_relative 'cop/migration/add_timestamps'
 require_relative 'cop/migration/datetime'
+require_relative 'cop/migration/hash_index'
 require_relative 'cop/migration/remove_concurrent_index'
 require_relative 'cop/migration/remove_index'
 require_relative 'cop/migration/reversible_add_column_with_default'

spec/rubocop/cop/migration/hash_index_spec.rb

+require 'spec_helper'
+
+require 'rubocop'
+require 'rubocop/rspec/support'
+
+require_relative '../../../../rubocop/cop/migration/hash_index'
+
+describe RuboCop::Cop::Migration::HashIndex do
+  include CopHelper
+
+  subject(:cop) { described_class.new }
+
+  context 'in migration' do
+    before do
+      allow(cop).to receive(:in_migration?).and_return(true)
+    end
+
+    it 'registers an offense when creating a hash index' do
+      inspect_source(cop, 'def change; add_index :table, :column, using: :hash; end')
+
+      aggregate_failures do
+        expect(cop.offenses.size).to eq(1)
+        expect(cop.offenses.map(&:line)).to eq([1])
+      end
+    end
+
+    it 'registers an offense when creating a concurrent hash index' do
+      inspect_source(cop, 'def change; add_concurrent_index :table, :column, using: :hash; end')
+
+      aggregate_failures do
+        expect(cop.offenses.size).to eq(1)
+        expect(cop.offenses.map(&:line)).to eq([1])
+      end
+    end
+
+    it 'registers an offense when creating a hash index using t.index' do
+      inspect_source(cop, 'def change; t.index :table, :column, using: :hash; end')
+
+      aggregate_failures do
+        expect(cop.offenses.size).to eq(1)
+        expect(cop.offenses.map(&:line)).to eq([1])
+      end
+    end
+  end
+
+  context 'outside of migration' do
+    it 'registers no offense' do
+      inspect_source(cop, 'def change; index :table, :column, using: :hash; end')
+
+      expect(cop.offenses.size).to eq(0)
+    end
+  end
+end