Add extension for Sequel transactions

We add a `Dry::Operation::Extensions::Sequel` module that, when included, gives access to a `#transaction` method. This method wraps the yielded steps in a [Sequel](https://sequel.jeremyevans.net/) transaction, rolling back in case one of them returns a failure.

The extension expects the including class to define a `#db` method giving access to the Sequel database definition:

```ruby
class MyOperation < Dry::Operation
  include Dry::Operation::Extensions::Sequel

  attr_reader :db

  def initialize(db:)
    @db = db
  end

  def call(input)
    attrs = step validate(input)
    user = transaction do
      new_user = step persist(attrs)
      step assign_initial_role(new_user)
      new_user
    end
    step notify(user)
    user
  end

  # ...
end
```

Default options for the `#transaction` options (which delegates to Sequel [transaction method](https://sequel.jeremyevans.net/rdoc/files/doc/transactions_rdoc.html)) can be given both at include time with `include Dry::Operation::Extensions::Sequel[isolation: :serializable]`, and at runtime with `#transaction(isolation: :serializable)`.

Gemfile

@@ -28,5 +28,6 @@ end
 group :development, :test do
   gem "activerecord"
   gem "rom-sql"
+  gem "sequel"
   gem "sqlite3", "~> 1.4"
 end

lib/dry/operation.rb

@@ -5,6 +5,7 @@
 require "dry/operation/errors"
 require "dry/operation/extensions/active_record"
 require "dry/operation/extensions/rom"
+require "dry/operation/extensions/sequel"
 
 module Dry
   # DSL for chaining operations that can fail

lib/dry/operation/extensions/sequel.rb

@@ -0,0 +1,102 @@
+# frozen_string_literal: true
+
+begin
+  require "sequel"
+rescue LoadError
+  raise Dry::Operation::MissingDependencyError.new(gem: "sequel", extension: "Sequel")
+end
+
+module Dry
+  class Operation
+    module Extensions
+      # Add Sequel transaction support to operations
+      #
+      # When this extension is included, you can use a `#transaction` method
+      # to wrap the desired steps in a Sequel transaction. If any of the steps
+      # returns a `Dry::Monads::Result::Failure`, the transaction will be rolled
+      # back and, as usual, the rest of the flow will be skipped.
+      #
+      # The extension expects the including class to give access to the Sequel
+      # database object via a `#db` method.
+      #
+      # ```ruby
+      # class MyOperation < Dry::Operation
+      #   include Dry::Operation::Extensions::Sequel
+      #
+      #   attr_reader :db
+      #
+      #   def initialize(db:)
+      #     @db = db
+      #   end
+      #
+      #   def call(input)
+      #     attrs = step validate(input)
+      #     user = transaction do
+      #       new_user = step persist(attrs)
+      #       step assign_initial_role(new_user)
+      #       new_user
+      #     end
+      #     step notify(user)
+      #     user
+      #   end
+      #
+      #   # ...
+      # end
+      # ```
+      #
+      # By default, no options are passed to the Sequel transaction. You can
+      # change this when including the extension:
+      #
+      # ```ruby
+      # include Dry::Operation::Extensions::Sequel[isolation: :serializable]
+      # ```
+      #
+      # Or you can change it at runtime:
+      #
+      # ```ruby
+      # transaction(isolation: :serializable) do
+      #   # ...
+      # end
+      # ```
+      #
+      # @see http://sequel.jeremyevans.net/rdoc/files/doc/transactions_rdoc.html
+      module Sequel
+        def self.included(klass)
+          klass.include(self[])
+        end
+
+        # Include the extension providing default options for the transaction.
+        #
+        # @param options [Hash] additional options for the Sequel transaction
+        def self.[](options = {})
+          Builder.new(**options)
+        end
+
+        # @api private
+        class Builder < Module
+          def initialize(**options)
+            super()
+            @options = options
+          end
+
+          def included(klass)
+            class_exec(@options) do |default_options|
+              klass.define_method(:transaction) do |**opts, &steps|
+                raise Dry::Operation::ExtensionError, <<~MSG unless respond_to?(:db)
+                  When using the Sequel extension, you need to define a #db method \
+                  that returns the Sequel database object
+                MSG
+
+                db.transaction(**default_options.merge(opts)) do
+                  intercepting_failure(-> { raise ::Sequel::Rollback }) do
+                    steps.()
+                  end
+                end
+              end
+            end
+          end
+        end
+      end
+    end
+  end
+end

spec/integration/extensions/sequel_spec.rb

@@ -0,0 +1,96 @@
+# frozen_string_literal: true
+
+require "spec_helper"
+
+RSpec.describe Dry::Operation::Extensions::Sequel do
+  include Dry::Monads[:result]
+
+  let(:db) do
+    Sequel.sqlite
+  end
+
+  before do
+    db.create_table(:users) do
+      primary_key :id
+      String :name
+    end
+  end
+
+  after do
+    db.drop_table(:users)
+  end
+
+  let(:base) do
+    Class.new(Dry::Operation) do
+      include Dry::Operation::Extensions::Sequel
+
+      attr_reader :db
+
+      def initialize(db:)
+        @db = db
+        super()
+      end
+    end
+  end
+
+  it "rolls transaction back on failure" do
+    instance = Class.new(base) do
+      def call
+        transaction do
+          step create_user
+          step failure
+        end
+      end
+
+      def create_user
+        Success(db[:users].insert(name: "John"))
+      end
+
+      def failure
+        Failure(:failure)
+      end
+    end.new(db: db)
+
+    instance.()
+    expect(db[:users].count).to be(0)
+  end
+
+  it "acts transparently for the regular flow" do
+    instance = Class.new(base) do
+      def call
+        transaction do
+          step create_user
+          step count_users
+        end
+      end
+
+      def create_user
+        Success(db[:users].insert(name: "John"))
+      end
+
+      def count_users
+        Success(db[:users].count)
+      end
+    end.new(db: db)
+
+    expect(instance.()).to eql(Success(1))
+  end
+
+  it "accepts options for Sequel transaction method" do
+    instance = Class.new(base) do
+      def call
+        transaction(isolation: :serializable) do
+          step create_user
+        end
+      end
+
+      def create_user
+        Success(db[:users].insert(name: "John"))
+      end
+    end.new(db: db)
+
+    expect(db).to receive(:transaction).with(isolation: :serializable)
+
+    instance.()
+  end
+end

spec/unit/extensions/sequel_spec.rb

@@ -0,0 +1,48 @@
+# frozen_string_literal: true
+
+require "spec_helper"
+
+RSpec.describe Dry::Operation::Extensions::Sequel do
+  describe "#transaction" do
+    it "raises a meaningful error when #db method is not implemented" do
+      instance = Class.new.include(Dry::Operation::Extensions::Sequel).new
+
+      expect { instance.transaction {} }.to raise_error(
+        Dry::Operation::ExtensionError,
+        /you need to define a #db method/
+      )
+    end
+
+    it "forwards options to Sequel transaction call" do
+      db = double(:db)
+      instance = Class.new do
+        include Dry::Operation::Extensions::Sequel
+
+        attr_reader :db
+
+        def initialize(db)
+          @db = db
+        end
+      end.new(db)
+
+      expect(db).to receive(:transaction).with(isolation: :serializable)
+      instance.transaction(isolation: :serializable) {}
+    end
+
+    it "merges options with default options" do
+      db = double(:db)
+      instance = Class.new do
+        include Dry::Operation::Extensions::Sequel[savepoint: true]
+
+        attr_reader :db
+
+        def initialize(db)
+          @db = db
+        end
+      end.new(db)
+
+      expect(db).to receive(:transaction).with(savepoint: true, isolation: :serializable)
+      instance.transaction(isolation: :serializable) {}
+    end
+  end
+end