Develop into master

.release-version

@@ -1 +1 @@
-14.43.0
+14.43.2

app/models/asset_link.rb

@@ -76,4 +76,81 @@ def has_#{name}?
       end
     end
   end
+
+  # Creates an edge between the ancestor and descendant nodes using save.
+  #
+  # This method first attempts to find an existing link between the ancestor
+  # and descendant. If no link is found, it builds a new edge and saves it.
+  # If a link is found, it makes the link an edge and saves it.
+  #
+  # This method is overridden to handle race conditions in finding an
+  # existing link and has_duplicates validation. It also assumes that there
+  # is a unique-together index on ancestor_id and descendant_id columns.
+  #
+  # @param ancestor [Dag::Standard::EndPoint] The ancestor node.
+  # @param descendant [Dag::Standard::EndPoint] The descendant node.
+  # @return [Boolean] Returns true if the edge is successfully created or
+  #   already exists, false otherwise.
+  # @raise [ActiveRecord::RecordNotUnique] Re-raises any exception if it is
+  #   not a constraint violation that involves ancestor_id and descendant_id
+  #   columns.
+  def self.create_edge(ancestor, descendant)
+    # Two processes try to find an existing link.
+    link = find_link(ancestor, descendant)
+    # Either or both may find no link and try to create a new edge.
+    if link.nil?
+      edge = build_edge(ancestor, descendant)
+      result = save_edge_or_handle_error(edge)
+      return result unless result.nil? # Bubble up.
+      # Losing process finds the edge created by the winning process.
+      link = find_link(ancestor, descendant)
+    end
+
+    return if link.nil?
+
+    link.make_direct
+    link.changed? ? link.save : true
+  end
+
+  # Saves the edge between the ancestor and descendant nodes or handles errors.
+  #
+  # @param edge [AssetLink] The edge object containing the errors.
+  # @return [Boolean] Returns true if the edge is successfully saved,
+  #   nil if the error is unique validation or constraint violation,
+  #   false if the error is another validation error.
+  # @raise [ActiveRecord::RecordNotUnique] Re-raises an exception if the
+  #   exception caught is not a unique constraint violation.
+  def self.save_edge_or_handle_error(edge)
+    # Winning process successfully saves the edge (direct link).
+    return true if edge.save
+    # has_duplicate validation may see it for the losing process before
+    # hitting the DB.
+    return false unless unique_validation_error?(edge) # Bubble up.
+    edge.errors.clear # Clear all errors and use the existing link.
+  rescue ActiveRecord::RecordNotUnique => e
+    # Unique constraint violation is triggered for the losing process after
+    # hitting the DB.
+    raise unless unique_violation_error?(edge, e) # Bubble up.
+  end
+
+  # Checks if the validation error includes a specific message indicating a
+  # unique link already exists.
+  #
+  # @param edge [AssetLink] The edge object containing the errors.
+  # @return [Boolean] Returns true if the errors include the message "Link
+  #   already exists between these points", false otherwise.
+  def self.unique_validation_error?(edge)
+    edge.errors[:base].include?('Link already exists between these points')
+  end
+
+  # Checks if the unique constraint violation involves the specified columns.
+  #
+  # @param edge [AssetLink] The edge object containing the column names.
+  # @param exception [ActiveRecord::RecordNotUnique] The exception raised due
+  #   to the unique constraint violation.
+  # @return [Boolean] Returns true if the exception message includes both the
+  #   ancestor and descendant column names, false otherwise.
+  def self.unique_violation_error?(edge, exception)
+    [edge.ancestor_id_column_name, edge.descendant_id_column_name].all? { |col| exception.message.include?(col) }
+  end
 end

db/migrate/20240912000109_add_unique_index_to_asset_links.rb

@@ -0,0 +1,26 @@
+# frozen_string_literal: true
+
+# This migration adds a unique-together index on ancestor_id and descendant_id
+# in order to prevent duplicate links between the same ancestor and descendant
+# labware.
+#
+# Before this migration, the database allowed duplicate asset_links between the
+# same ancestor and descendant labware. Therefore, the migration will fail if
+# there are any duplicate links in the database. To fix this, they must be
+# removed before running the migration using the rake task:
+#
+# bundle exec rake 'support:remove_duplicate_asset_links[csv_file_path]'
+#
+# The rake task will write the removed records into a CSV file that can be used
+# for auditing purposes.
+#
+# Note that the column names in the index name below is used for finding the
+# reason of the database unique constraint violation by the AssetLink model.
+class AddUniqueIndexToAssetLinks < ActiveRecord::Migration[6.1]
+  def change
+    add_index :asset_links,
+              %i[ancestor_id descendant_id],
+              unique: true,
+              name: 'index_asset_links_on_ancestor_id_and_descendant_id'
+  end
+end

db/schema.rb

@@ -115,6 +115,7 @@
     t.integer "count"
     t.datetime "created_at", null: false
     t.datetime "updated_at", null: false
+    t.index ["ancestor_id", "descendant_id"], name: "index_asset_links_on_ancestor_id_and_descendant_id", unique: true
     t.index ["ancestor_id", "direct"], name: "index_asset_links_on_ancestor_id_and_direct"
     t.index ["descendant_id", "direct"], name: "index_asset_links_on_descendant_id_and_direct"
   end