5 files changed, 94 insertions, 1 deletions
diff --git a/app/models/concerns/noteable.rb b/app/models/concerns/noteable.rb
index eb315058c3a..f2cad09e779 100644
--- a/app/models/concerns/noteable.rb
+++ b/app/models/concerns/noteable.rb
@@ -26,6 +26,10 @@ module Noteable
     DiscussionNote.noteable_types.include?(base_class_name)
   end
 
+  def supports_suggestion?
+    false
+  end
+
   def discussions_rendered_on_frontend?
     false
   end
diff --git a/app/models/diff_note.rb b/app/models/diff_note.rb
index c32008aa9c7..279603496b0 100644
--- a/app/models/diff_note.rb
+++ b/app/models/diff_note.rb
@@ -66,10 +66,23 @@ class DiffNote < Note
     self.original_position.diff_refs == diff_refs
   end
 
+  def supports_suggestion?
+    return false unless noteable.supports_suggestion? && on_text?
+    # We don't want to trigger side-effects of `diff_file` call.
+    return false unless file = fetch_diff_file
+    return false unless line = file.line_for_position(self.original_position)
+
+    line&.suggestible?
+  end
+
   def discussion_first_note?
     self == discussion.first_note
   end
 
+  def banzai_render_context(field)
+    super.merge(suggestions_filter_enabled: supports_suggestion?)
+  end
+
   private
 
   def enqueue_diff_file_creation_job
diff --git a/app/models/merge_request.rb b/app/models/merge_request.rb
index a13cac73d04..8052a54c504 100644
--- a/app/models/merge_request.rb
+++ b/app/models/merge_request.rb
@@ -363,6 +363,11 @@ class MergeRequest < ActiveRecord::Base
     end
   end
 
+  def supports_suggestion?
+    # Should be `true` when removing the FF.
+    Suggestion.feature_enabled?
+  end
+
   # Calls `MergeWorker` to proceed with the merge process and
   # updates `merge_jid` with the MergeWorker#jid.
   # This helps tracking enqueued and ongoing merge jobs.
diff --git a/app/models/note.rb b/app/models/note.rb
index 17c7d97fa0a..becf14e9785 100644
--- a/app/models/note.rb
+++ b/app/models/note.rb
@@ -69,6 +69,12 @@ class Note < ActiveRecord::Base
   belongs_to :last_edited_by, class_name: 'User'
 
   has_many :todos
+
+  # The delete_all definition is required here in order
+  # to generate the correct DELETE sql for
+  # suggestions.delete_all calls
+  has_many :suggestions, -> { order(:relative_order) },
+    inverse_of: :note, dependent: :delete_all # rubocop:disable Cop/ActiveRecordDependent
   has_many :events, as: :target, dependent: :destroy # rubocop:disable Cop/ActiveRecordDependent
   has_one :system_note_metadata
   has_one :note_diff_file, inverse_of: :diff_note, foreign_key: :diff_note_id
@@ -110,7 +116,7 @@ class Note < ActiveRecord::Base
   scope :inc_author, -> { includes(:author) }
   scope :inc_relations_for_view, -> do
     includes(:project, { author: :status }, :updated_by, :resolved_by, :award_emoji,
-             :system_note_metadata, :note_diff_file)
+             :system_note_metadata, :note_diff_file, :suggestions)
   end
 
   scope :with_notes_filter, -> (notes_filter) do
@@ -226,6 +232,10 @@ class Note < ActiveRecord::Base
     Gitlab::HookData::NoteBuilder.new(self).build
   end
 
+  def supports_suggestion?
+    false
+  end
+
   def for_commit?
     noteable_type == "Commit"
   end
diff --git a/app/models/suggestion.rb b/app/models/suggestion.rb
new file mode 100644
index 00000000000..cec5ea30f9d
--- /dev/null
+++ b/app/models/suggestion.rb
@@ -0,0 +1,61 @@
+# frozen_string_literal: true
+
+class Suggestion < ApplicationRecord
+  FEATURE_FLAG = :diff_suggestions
+
+  belongs_to :note, inverse_of: :suggestions
+  validates :note, presence: true
+  validates :commit_id, presence: true, if: :applied?
+
+  delegate :original_position, :position, :diff_file,
+    :noteable, to: :note
+
+  def self.feature_enabled?
+    Feature.enabled?(FEATURE_FLAG)
+  end
+
+  def project
+    noteable.source_project
+  end
+
+  def branch
+    noteable.source_branch
+  end
+
+  # For now, suggestions only serve as a way to send patches that
+  # will change a single line (being able to apply multiple in the same place),
+  # which explains `from_line` and `to_line` being the same line.
+  # We'll iterate on that in https://gitlab.com/gitlab-org/gitlab-ce/issues/53310
+  # when allowing multi-line suggestions.
+  def from_line
+    position.new_line
+  end
+  alias_method :to_line, :from_line
+
+  def from_original_line
+    original_position.new_line
+  end
+  alias_method :to_original_line, :from_original_line
+
+  # `from_line_index` and `to_line_index` represents diff/blob line numbers in
+  # index-like way (N-1).
+  def from_line_index
+    from_line - 1
+  end
+  alias_method :to_line_index, :from_line_index
+
+  def appliable?
+    return false unless note.supports_suggestion?
+
+    !applied? &&
+      noteable.opened? &&
+      different_content? &&
+      note.active?
+  end
+
+  private
+
+  def different_content?
+    from_content != to_content
+  end
+end