Allow Dry::Validation::Contract rule definitions in Hanami::Action

lib/hanami/action/params.rb

@@ -148,6 +148,35 @@ def self.params(&blk)
         validations(&blk || -> {})
       end
 
+      # Define a validation rule.
+      #
+      # @param blk [Proc] the rule definition
+      #
+      # @since 2.x.x
+      #
+      # @example
+      #   class Event < Hanami::Action
+      #     params do
+      #       required(:start_date).value(:date)
+      #       required(:end_date).value(:date)
+      #     end
+      #
+      #     # Rules must be defined after the params schema.
+      #     rule(:start_date, :end_date) do
+      #       if start_date > end_date
+      #         base.failure('event cannot end before it starts')
+      #       end
+      #     end
+      #
+      #     def handle(req, *)
+      #       halt 400 unless req.params.valid?
+      #       # ...
+      #     end
+      #   end
+      def self.rule(...)
+        _validator.class.rule(...)
+      end
+
       # Initialize the params and freeze them.
       #
       # @param env [Hash] a Rack env or an hash of params.

lib/hanami/action/validatable.rb

@@ -105,6 +105,40 @@ def params(klass = nil, &blk)
 
           @params_class = klass
         end
+
+        # Define a validation rule.
+        #
+        # @param blk [Proc] the rule definition
+        #
+        # @since 2.x.x
+        #
+        # @example
+        #   class Event < Hanami::Action
+        #     params do
+        #       required(:start_date).value(:date)
+        #       required(:end_date).value(:date)
+        #     end
+        #
+        #     # Rules must be defined after the params schema.
+        #     rule(:start_date, :end_date) do
+        #       if start_date > end_date
+        #         base.failure('event cannot end before it starts')
+        #       end
+        #     end
+        #
+        #     def handle(req, *)
+        #       halt 400 unless req.params.valid?
+        #       # ...
+        #     end
+        #   end
+        def rule(...)
+          unless @params_class
+            raise ArgumentError.new(
+              "The params schema must be defined prior to adding rules."
+            )
+          end
+          @params_class.class_eval { rule(...) }
+        end
       end
     end
   end

spec/unit/hanami/action/rules_spec.rb

@@ -0,0 +1,87 @@
+# frozen_string_literal: true
+
+require "rack"
+
+RSpec.describe Hanami::Action do
+  describe ".params" do
+    let(:klass) do
+      Class.new(described_class) do
+        params do
+          required(:book).schema do
+            required(:code).filled(:str?)
+          end
+        end
+
+        def handle(request, response)
+          response[:params] = request.params
+        end
+      end
+    end
+
+    let(:action) { klass.new }
+    let(:response) { action.call(given_input) }
+
+    context "given valid input" do
+      let(:given_input) { {book: {code: "abc"}} }
+
+      it "is valid" do
+        expect(response[:params].valid?).to eq(true)
+      end
+    end
+
+    context "given invalid input" do
+      let(:given_input) { {book: {code: nil}} }
+
+      it "is not valid" do
+        expect(response[:params].valid?).to eq(false)
+      end
+    end
+  end
+
+  describe ".rule" do
+    let(:klass) do
+      Class.new(described_class) do
+        params do
+          required(:book).schema do
+            required(:code).filled(:str?)
+          end
+        end
+
+        rule("book.code") do
+          key.failure('must be "abc"') unless value == "abc"
+        end
+
+        def handle(request, response)
+          response[:params] = request.params
+        end
+      end
+    end
+
+    let(:action) { klass.new }
+    let(:response) { action.call(given_input) }
+
+    context "given valid input" do
+      let(:given_input) { {book: {code: "abc"}} }
+
+      it "is valid" do
+        expect(response[:params].valid?).to eq(true)
+      end
+    end
+
+    context "given invalid input" do
+      let(:given_input) { {book: {code: nil}} }
+
+      it "is not valid" do
+        expect(response[:params].valid?).to eq(false)
+      end
+    end
+
+    context "given input which does not pass rules" do
+      let(:given_input) { {book: {code: "xyz"}} }
+
+      it "is not valid" do
+        expect(response[:params].valid?).to eq(false)
+      end
+    end
+  end
+end