New cop rule: IrreversibleMigration

config/default.yml

@@ -11,6 +11,14 @@ Sequel/ConcurrentIndex:
   VersionAdded: 0.0.1
   VersionChanged: 0.3.3
 
+Sequel/IrreversibleMigration:
+  Description: >-
+    Warns against using certain methods inside a migration file's `change` block that would cause the migration to become irreversible.
+  Reference: https://www.rubydoc.info/gems/rubocop-sequel/RuboCop/Cop/Sequel/IrreversibleMigration
+  Enabled: true
+  VersionAdded: 0.3.4
+  VersionChanged: 0.3.4
+
 Sequel/JSONColumn:
   Description: >-
     Advocates the use of the `jsonb` column type over `json` or `hstore` for performance

lib/rubocop-sequel.rb

@@ -8,6 +8,7 @@
 RuboCop::Sequel::Inject.defaults!
 
 require 'rubocop/cop/sequel/concurrent_index'
+require 'rubocop/cop/sequel/irreversible_migration'
 require 'rubocop/cop/sequel/json_column'
 require 'rubocop/cop/sequel/migration_name'
 require 'rubocop/cop/sequel/save_changes'

lib/rubocop/cop/sequel/irreversible_migration.rb

@@ -0,0 +1,57 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Sequel
+      # IrreversibleMigration looks for methods inside a `change` block that cannot be reversed.
+      class IrreversibleMigration < Base
+        # https://sequel.jeremyevans.net/rdoc/files/doc/migration_rdoc.html#label-A+Basic+Migration
+        VALID_CHANGE_METHODS = %i[
+          create_table
+          create_join_table
+          create_view
+          add_column
+          add_index
+          rename_column
+          rename_table
+          alter_table
+          add_column
+          add_constraint
+          add_foreign_key
+          add_primary_key
+          add_index
+          add_full_text_index
+          add_spatial_index
+          rename_column
+          set_column_allow_null
+        ].freeze
+
+        MSG = 'Avoid using `%<name>s` inside a `change` block. Use `up` & `down` blocks instead.'
+        PRIMARY_KEY_MSG = 'Avoid using `add_primary_key` with an array argument inside a `change` block.'
+
+        def on_block(node)
+          return unless node.method_name == :change
+
+          body = node.body
+          return unless body
+
+          body.each_node(:send) { |child_node| validate_node(child_node) }
+        end
+
+        private
+
+        def validate_node(node)
+          name = node.method_name
+
+          add_offense(node.loc.selector, message: format(MSG, name: name)) unless VALID_CHANGE_METHODS.include?(name)
+
+          return unless name == :add_primary_key
+
+          return unless node.arguments.any?(&:array_type?)
+
+          add_offense(node.loc.selector, message: PRIMARY_KEY_MSG)
+        end
+      end
+    end
+  end
+end

rubocop-sequel.gemspec

@@ -17,5 +17,5 @@ Gem::Specification.new do |gem|
 
   gem.required_ruby_version = '>= 2.5'
 
-  gem.add_runtime_dependency 'rubocop', '~> 1.0'
+  gem.add_dependency 'rubocop', '~> 1.0'
 end

spec/rubocop/cop/sequel/irreversible_migration_spec.rb

@@ -0,0 +1,76 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::Sequel::IrreversibleMigration do
+  subject(:cop) { described_class.new }
+
+  context 'when inside a change block' do
+    let(:invalid_source) do
+      <<~SOURCE
+        change do
+          alter_table(:stores) do
+            drop_column(:products, :name)
+            drop_index(:products, :price)
+          end
+        end
+      SOURCE
+    end
+
+    let(:valid_source) do
+      <<~SOURCE
+        change do
+          alter_table(:stores) do
+            add_primary_key(:id)
+            add_column(:products, :name)
+            add_index(:products, :price)
+          end
+        end
+      SOURCE
+    end
+
+    it 'registers an offense when there is an invalid method' do
+      offenses = inspect_source(invalid_source)
+      expect(offenses.size).to eq(2)
+    end
+
+    it 'does not register an offense with valid methods' do
+      offenses = inspect_source(valid_source)
+      expect(offenses).to be_empty
+    end
+
+    describe 'and an array is passed into `add_primary_key`' do
+      let(:source) do
+        <<~SOURCE
+          change do
+            alter_table(:stores) do
+              add_primary_key([:owner_id, :name])
+            end
+          end
+        SOURCE
+      end
+
+      it 'registers an offense' do
+        offenses = inspect_source(source)
+        expect(offenses.size).to eq(1)
+      end
+    end
+  end
+
+  context 'when inside an up block' do
+    let(:source) do
+      <<~SOURCE
+        up do
+          alter_table(:stores) do
+            add_primary_key([:owner_id, :name])
+            add_column(:products, :name)
+            drop_index(:products, :price)
+          end
+        end
+      SOURCE
+    end
+
+    it 'does not register an offense with any methods' do
+      offenses = inspect_source(source)
+      expect(offenses).to be_empty
+    end
+  end
+end