Add DB::Serializable (#115)

src/db.cr

@@ -199,3 +199,4 @@ require "./db/pool_unprepared_statement"
 require "./db/result_set"
 require "./db/error"
 require "./db/mapping"
+require "./db/serializable"

src/db/serializable.cr

@@ -0,0 +1,180 @@
+module DB
+  annotation Field
+  end
+
+  # The `DB::Serialization` module automatically generates methods for DB serialization when included.
+  #
+  # Once included, `ResultSet#read(t)` populates properties of the class from the
+  # `ResultSet`.
+  #
+  # ### Example
+  #
+  # ```crystal
+  # require "db"
+  #
+  # class Employee
+  #   include DB::Serializable
+  #
+  #   property title : String
+  #   property name : String
+  # end
+  #
+  # employees = Employee.from_rs(db.query("SELECT title, name FROM employees"))
+  # employees[0].title # => "Manager"
+  # employees[0].name  # => "John"
+  # ```
+  #
+  # ### Usage
+  #
+  # `DB::Serializable` was designed in analogue with `JSON::Serializable`, so usage is identical.
+  #  However, like `DB.mapping`, `DB::Serializable` is **strict by default**, so extra columns will raise `DB::MappingException`s.
+  #
+  # Similar to `JSON::Field`, there is an annotation `DB::Field` that can be used to set serialization behavior
+  # on individual instance variables.
+  #
+  # ```crystal
+  # class Employee
+  #   include DB::Serializable
+  #
+  #   property title : String
+  #
+  #   @[DB::Field(key: "firstname")]
+  #   property name : String?
+  # end
+  # ```
+  #
+  # `DB::Field` properties:
+  # * **ignore**: if `true`, skip this field in serialization and deserialization (`false` by default)
+  # * **key**: defines which column to read from a `ResultSet` (name of the instance variable by default)
+  # * **converter**: defines an alternate type for parsing results. The given type must define `#from_rs(DB::ResultSet)` and return an instance of the included type.
+  #
+  # ### `DB::Serializable::NonStrict`
+  #
+  # Including this module is functionally identical to passing `{strict: false}` to `DB.mapping`: extra columns will not raise.
+  #
+  # ```crystal
+  # class Employee
+  #   include DB::Serializable
+  #   include DB::Serializable::NonStrict
+  #
+  #   property title : String
+  #   property name : String
+  # end
+  #
+  # # does not raise!
+  # employees = Employee.from_rs(db.query("SELECT title, name, age FROM employees"))
+  # ```
+  module Serializable
+    macro included
+      include ::DB::Mappable
+
+      # Define a `new` and `from_rs` directly in the type, like JSON::Serializable
+      # For proper overload resolution
+
+      def self.new(rs : ::DB::ResultSet)
+        instance = allocate
+        instance.initialize(__set_for_db_serializable: rs)
+        GC.add_finalizer(instance) if instance.responds_to?(:finalize)
+        instance
+      end
+
+      def self.from_rs(rs : ::DB::ResultSet)
+        objs = Array(self).new
+        rs.each do
+          objs << self.new(rs)
+        end
+        objs
+      ensure
+        rs.close
+      end
+
+      # Inject the class methods into subclasses as well
+
+      macro inherited
+        def self.new(rs : ::DB::ResultSet)
+          super
+        end
+
+        def self.from_rs(rs : ::DB::Result_set)
+          super
+        end
+      end
+    end
+
+    def initialize(*, __set_for_db_serializable rs : ::DB::ResultSet)
+      {% begin %}
+        {% properties = {} of Nil => Nil %}
+        {% for ivar in @type.instance_vars %}
+          {% ann = ivar.annotation(::DB::Field) %}
+          {% unless ann && ann[:ignore] %}
+            {%
+              properties[ivar.id] = {
+                type:      ivar.type,
+                key:       ((ann && ann[:key]) || ivar).id.stringify,
+                default:   ivar.default_value,
+                nilable:   ivar.type.nilable?,
+                converter: ann && ann[:converter],
+              }
+            %}
+          {% end %}
+        {% end %}
+
+        {% for name, value in properties %}
+          %var{name} = nil
+          %found{name} = false
+        {% end %}
+
+        rs.each_column do |col_name|
+          case col_name
+            {% for name, value in properties %}
+              when {{value[:key]}}
+                %found{name} = true
+                %var{name} =
+                  {% if value[:converter] %}
+                    {{value[:converter]}}.from_rs(rs)
+                  {% elsif value[:nilable] || value[:default] != nil %}
+                    rs.read(::Union({{value[:type]}} | Nil))
+                  {% else %}
+                    rs.read({{value[:type]}})
+                  {% end %}
+            {% end %}
+          else
+            rs.read # Advance set, but discard result
+            on_unknown_db_column(col_name)
+          end
+        end
+
+        {% for key, value in properties %}
+          {% unless value[:nilable] || value[:default] != nil %}
+            if %var{key}.is_a?(Nil) && !%found{key}
+              raise ::DB::MappingException.new("missing result set attribute: {{(value[:key] || key).id}}")
+            end
+          {% end %}
+        {% end %}
+
+        {% for key, value in properties %}
+          {% if value[:nilable] %}
+            {% if value[:default] != nil %}
+              @{{key}} = %found{key} ? %var{key} : {{value[:default]}}
+            {% else %}
+              @{{key}} = %var{key}
+            {% end %}
+          {% elsif value[:default] != nil %}
+            @{{key}} = %var{key}.is_a?(Nil) ? {{value[:default]}} : %var{key}
+          {% else %}
+            @{{key}} = %var{key}.as({{value[:type]}})
+          {% end %}
+        {% end %}
+      {% end %}
+    end
+
+    protected def on_unknown_db_column(col_name)
+      raise ::DB::MappingException.new("unknown result set attribute: #{col_name}")
+    end
+
+    module NonStrict
+      protected def on_unknown_db_column(col_name)
+      end
+    end
+  end
+end