Support automatic schema inferring

Gemfile

@@ -15,7 +15,6 @@ group :test do
   gem 'anima', '~> 0.2.0'
   gem 'minitest'
   gem 'thread_safe'
-  gem 'activesupport'
   gem 'inflecto', '~> 0.0', '>= 0.0.2'
 
   platforms :rbx do

lib/rom/relation/class_interface.rb

@@ -31,10 +31,11 @@ def inherited(klass)
         klass.class_eval do
           use :registry_reader
 
-          defines :gateway, :dataset, :dataset_proc, :register_as, :schema_dsl
+          defines :gateway, :dataset, :dataset_proc, :register_as, :schema_dsl, :schema_inferrer
 
           gateway :default
           schema_dsl Schema::DSL
+          schema_inferrer nil
 
           dataset default_name
 
@@ -146,13 +147,16 @@ def [](adapter)
       # @return [Schema]
       #
       # @param [Symbol] dataset An optional dataset name
+      # @param [Boolean] infer Whether to do an automatic schema inferring
       #
       # @api public
-      def schema(dataset = nil, &block)
+      def schema(dataset = nil, infer: false, &block)
         if defined?(@schema)
           @schema
-        elsif block
-          @schema = schema_dsl.new(dataset || self.dataset, &block).call
+        elsif block || infer
+          @schema = schema_dsl.new(dataset || self.dataset,
+                                   inferrer: infer ? schema_inferrer : nil,
+                                   &block).call
 
           if dataset
             self.dataset(dataset)

lib/rom/schema.rb

@@ -6,20 +6,27 @@ module ROM
   #
   # @api public
   class Schema
-    include Dry::Equalizer(:dataset, :attributes, :meta)
+    include Dry::Equalizer(:dataset, :attributes, :inferrer)
     include Enumerable
 
-    attr_reader :dataset, :attributes, :meta
+    attr_reader :dataset, :attributes, :inferrer
 
     # @api public
     class DSL < BasicObject
-      attr_reader :dataset, :attributes
+      attr_reader :dataset, :attributes, :inferrer
 
       # @api private
-      def initialize(dataset = nil, &block)
-        @attributes = {}
+      def initialize(dataset = nil, inferrer: nil, &block)
+        @attributes = nil
         @dataset = dataset
-        instance_exec(&block)
+        @inferrer = inferrer
+
+        if block
+          instance_exec(&block)
+        elsif inferrer.nil?
+          raise ArgumentError,
+                'You must pass a block to define a schema or set an inferrer for automatic inferring'
+        end
       end
 
       # Defines a relation attribute with its type
@@ -28,6 +35,7 @@ def initialize(dataset = nil, &block)
       #
       # @api public
       def attribute(name, type)
+        @attributes ||= {}
         @attributes[name] = type.meta(name: name)
       end
 
@@ -43,15 +51,17 @@ def primary_key(*names)
 
       # @api private
       def call
-        Schema.new(dataset, attributes)
+        Schema.new(dataset, attributes, inferrer: inferrer && inferrer.new(self))
       end
     end
 
     # @api private
-    def initialize(dataset, attributes)
+    def initialize(dataset, attributes, inferrer: nil)
       @dataset = dataset
       @attributes = attributes
-      freeze
+      @inferrer = inferrer
+
+      freeze if self.defined?
     end
 
     # Iterate over schema's attributes
@@ -79,5 +89,16 @@ def primary_key
     def foreign_key(relation)
       detect { |attr| attr.meta[:foreign_key] && attr.meta[:relation] == relation }
     end
+
+    # @api public
+    def defined?
+      !@attributes.nil?
+    end
+
+    # @api private
+    def infer!(gateway)
+      @attributes = inferrer.call(dataset, gateway)
+      freeze
+    end
   end
 end

lib/rom/setup/finalize/finalize_relations.rb

@@ -27,6 +27,10 @@ def run!
           #       where klass' gateway points to non-existant repo
           gateway = @gateways.fetch(klass.gateway)
           ds_proc = klass.dataset_proc || -> _ { self }
+
+          if klass.schema && !klass.schema.defined?
+            klass.schema.infer!(gateway)
+          end
           dataset = gateway.dataset(klass.dataset).instance_exec(klass, &ds_proc)
 
           relation = klass.new(dataset, __registry__: registry)