Introduce new models TransformationMapping and TransformationMappingItem

Create a complete mapping through a virtual column.

app/models/transformation_mapping.rb

@@ -0,0 +1,61 @@
+class TransformationMapping < ApplicationRecord
+  has_many :transformation_mapping_items, :dependent => :destroy
+
+  validates :name, :presence => true, :uniqueness => true
+
+  virtual_column :mappings, :type => :string
+
+  # forward compatible: next release the table will be polymorphic
+  virtual_column :type,     :type => :string
+
+  def self.inheritance_column
+    "none"
+  end
+
+  def type
+    "MigrationInfrastructureMapping"
+  end
+
+  def type=(type)
+  end
+
+  # Assumption: source and destination types are the same. Works for infrastructure mapping
+  # group mapping items first by destination_type, then by destination_id
+  # simplify records with IDs.
+  # Sample result (Hash)
+  #
+  # EmsCluster:
+  #   - destination: "2":
+  #     sources: ["100", "200"]
+  #   - destination: "3"
+  #     sources: ["301", "302"]
+  # Network:
+  #   - destination: "2"
+  #     sources: ["202", "204"]
+  def mappings
+    dst_type_groups = transformation_mapping_items.group_by(&:destination_type)
+    dst_type_groups.each_with_object({}) do |(dst_type, items), results|
+      dst_groups = items.group_by { |item| item.destination_id.to_s }
+      results[dst_type] = dst_groups.each_with_object({}) do |(dst_id, item_array), dst_id_groups|
+        dst_id_groups["destination"] = dst_id
+        dst_id_groups["sources"] = item_array.collect { |item| item.source_id.to_s }
+      end
+    end
+  end
+
+  def mappings=(options)
+    options.each do |dst_type, item_map_array|
+      item_map_array.each do |item_map|
+        destination_id = item_map['destination']
+        item_map['sources'].each do |source_id|
+          transformation_mapping_items << TransformationMappingItem.new(
+            :source_id        => source_id,
+            :source_type      => dst_type,
+            :destination_id   => destination_id,
+            :destination_type => dst_type
+          )
+        end
+      end
+    end
+  end
+end

app/models/transformation_mapping_item.rb

@@ -0,0 +1,7 @@
+class TransformationMappingItem < ApplicationRecord
+  belongs_to :transformation_mapping
+  belongs_to :source,      :polymorphic => true
+  belongs_to :destination, :polymorphic => true
+
+  validates :source_id, :uniqueness => {:scope => [:transformation_mapping_id, :source_type]}
+end