FEATURE: Add CommentMigration for db column comments

For documenting the database (will be used in discourse-data-explorer)

lib/comment_migration.rb

@@ -0,0 +1,58 @@
+
+
+class CommentMigration < ActiveRecord::Migration
+  def comments_up
+    raise "Not implemented"
+  end
+
+  def up
+    comments_up.each do |table|
+      table[1].each do |column|
+        table_name = table[0]
+        column_name = column[0]
+        comment = column[1]
+
+        if column_name == :_table
+          ActiveRecord::Base.exec_sql "COMMENT ON TABLE #{table_name} IS ?", comment
+          puts "  COMMENT ON TABLE #{table_name}"
+        else
+          ActiveRecord::Base.exec_sql "COMMENT ON COLUMN #{table_name}.#{column_name} IS ?", comment
+          puts "  COMMENT ON COLUMN #{table_name}.#{column_name}"
+        end
+      end
+    end
+  end
+
+  def comments_down
+    {}
+  end
+
+  def down
+    replace_nils(comments_up).deep_merge(comments_down).each do |table|
+      table[1].each do |column|
+        table_name = table[0]
+        column_name = column[0]
+        comment = column[1]
+
+        if column_name == :_table
+          ActiveRecord::Base.exec_sql "COMMENT ON TABLE #{table_name} IS ?", comment
+          puts "  COMMENT ON TABLE #{table_name}"
+        else
+          ActiveRecord::Base.exec_sql "COMMENT ON COLUMN #{table_name}.#{column_name} IS ?", comment
+          puts "  COMMENT ON COLUMN #{table_name}.#{column_name}"
+        end
+      end
+    end
+  end
+
+  private
+  def replace_nils(hash)
+    hash.each do |key, value|
+      if Hash === value
+        hash[key] = replace_nils value
+      else
+        hash[key] = nil
+      end
+    end
+  end
+end