Allow for TabulatrData inheritance

One can inherit and expand TabulatrData for a given relation,
e.g. by adding extra columns and/or filters
in addition to those in parent class.

lib/tabulatr/data/data.rb

@@ -104,11 +104,11 @@ def execute_batch_actions batch_param, selected_ids
   end
 
   def table_columns
-    self.class.instance_variable_get("@table_columns")
+    collect :@table_columns
   end
 
   def filters
-    self.class.instance_variable_get('@filters')
+    collect :@filters
   end
 
   def search?
@@ -153,6 +153,17 @@ def join_required_tables(params)
     # @relation = @relation.group("#{@table_name}.#{@base.primary_key}")
   end
 
+  private
+
+  def collect(variable)
+    self.class.ancestors.inject([]) do |cols, tdc|
+      if tdc < Tabulatr::Data
+        tc = tdc.instance_variable_get variable
+        cols += tc if tc
+      end
+      cols
+    end
+  end
 end
 
 require_relative './dsl'

lib/tabulatr/data/dsl.rb

@@ -144,7 +144,7 @@ def filter(name, partial: nil, &block)
 
   def target_class(name)
     s = name.to_s
-    @target_class = s.camelize.constantize rescue "There's no class `#{s.camelize}' for `#{self.name}'"
+    @target_class = s.camelize.safe_constantize
     @target_class_name = s.underscore
   end
 

lib/tabulatr/renderer/renderer.rb

@@ -123,12 +123,11 @@ def get_requested_filters(available_filters, requested_filters)
     end.flatten
   end
 
-  def get_data_class(name = '')
-    if name.present?
-      name.constantize.new(@klass)
-    else
-      "#{@klass.name}TabulatrData".constantize.new(@klass)
-    end
+  def get_data_class(name = nil)
+    name = "#{@klass.name}TabulatrData" if name.blank?
+    name = name.constantize if name.is_a?(String)
+
+    name.new(@klass)
   end
 
   def set_columns_and_filters(data_class, columns, filters, &block)