Add validation to ensure child can't be linked to multiple parents

Resolves https://github.com/alphagov/whitehall/pull/11431/changes#r3258493572

Author: ChrisBAshton

app/models/parent_child_relationship.rb

@@ -12,6 +12,7 @@ class ParentChildRelationship < ApplicationRecord
             uniqueness: { scope: :parent_edition_id }
 
   validate :parent_must_be_prepublication, on: :create
+  validate :child_document_must_have_no_other_parent, on: :create
 
 private
 
@@ -24,4 +25,16 @@ def parent_must_be_prepublication
       errors.add(:parent_edition, "does not support child documents")
     end
   end
+
+  def child_document_must_have_no_other_parent
+    return if parent_edition.blank? || child_document.blank? # These are already covered by presence validation
+
+    existing_relationships = ParentChildRelationship.where(child_document_id: child_document_id)
+    if existing_relationships.count.positive?
+      document_ids = existing_relationships.map { |r| r.parent_edition.document.id }.uniq
+      unless parent_edition.document.id.in?(document_ids)
+        errors.add(:child_document, "is already linked to a different parent document")
+      end
+    end
+  end
 end

test/unit/app/models/parent_child_relationship_test.rb

@@ -40,6 +40,31 @@ class ParentChildRelationshipTest < ActiveSupport::TestCase
     assert_not relationship.valid?
   end
 
+  test "should be invalid if child has existing relationship with another parent" do
+    parent_document_one = create(:document)
+    parent_document_two = create(:document)
+    parent_edition_one = create(:draft_standard_edition, document: parent_document_one, configurable_document_type: "parent_type")
+    parent_edition_two = create(:draft_standard_edition, document: parent_document_two, configurable_document_type: "parent_type")
+    child_document = create(:document)
+
+    existing_relationship = create(
+      :parent_child_relationship,
+      parent_edition: parent_edition_one,
+      child_document:,
+    )
+    additional_relationship = build(
+      :parent_child_relationship,
+      parent_edition: parent_edition_two,
+      child_document:,
+    )
+    assert existing_relationship.valid?
+    assert_not additional_relationship.valid?
+    assert_includes(
+      additional_relationship.errors[:child_document],
+      "is already linked to a different parent document",
+    )
+  end
+
   test "should be valid if one parent edition has two separate child documents" do
     parent_edition = @valid_parent_edition