1 files changed, 55 insertions, 0 deletions
diff --git a/app/models/concerns/issuable_link.rb b/app/models/concerns/issuable_link.rb
new file mode 100644
index 00000000000..3e14507bc70
--- /dev/null
+++ b/app/models/concerns/issuable_link.rb
@@ -0,0 +1,55 @@
+# frozen_string_literal: true
+
+# == IssuableLink concern
+#
+# Contains common functionality shared between related Issues and related Epics
+#
+# Used by IssueLink, Epic::RelatedEpicLink
+#
+module IssuableLink
+  extend ActiveSupport::Concern
+
+  TYPE_RELATES_TO = 'relates_to'
+  TYPE_BLOCKS = 'blocks' ## EE-only. Kept here to be used on link_type enum.
+
+  class_methods do
+    def inverse_link_type(type)
+      type
+    end
+
+    def issuable_type
+      raise NotImplementedError
+    end
+  end
+
+  included do
+    validates :source, presence: true
+    validates :target, presence: true
+    validates :source, uniqueness: { scope: :target_id, message: 'is already related' }
+    validate :check_self_relation
+    validate :check_opposite_relation
+
+    enum link_type: { TYPE_RELATES_TO => 0, TYPE_BLOCKS => 1 }
+
+    private
+
+    def check_self_relation
+      return unless source && target
+
+      if source == target
+        errors.add(:source, 'cannot be related to itself')
+      end
+    end
+
+    def check_opposite_relation
+      return unless source && target
+
+      if self.class.base_class.find_by(source: target, target: source)
+        errors.add(:source, "is already related to this #{self.class.issuable_type}")
+      end
+    end
+  end
+end
+
+IssuableLink.prepend_mod_with('IssuableLink')
+IssuableLink::ClassMethods.prepend_mod_with('IssuableLink::ClassMethods')