Raise error on invalid fixture primary key

When you're using a custom primary key on a belongs_to and you're trying to load that value through the association shorthand in a fixture, you end up getting the primary key of the table and not the primary key specified in the join. This makes sense to keep as the behavior because it's super fast (just hashing the name of the fixture), but it's still surprising so we should want the developer that it's not possible to do what they want.

activerecord/lib/active_record/fixture_set/table_row.rb

@@ -39,6 +39,29 @@ module ActiveRecord
         end
       end
 
+      class PrimaryKeyError < StandardError # :nodoc:
+        def initialize(label, association, value)
+          super(<<~MSG)
+            Unable to set #{association.name} to #{value} because the association has a
+            custom primary key (#{association.join_primary_key}) that does not match the
+            associated table's primary key (#{association.klass.primary_key}).
+
+            To fix this, change your fixture from
+
+            #{label}:
+              #{association.name}: #{value}
+
+            to
+
+            #{label}:
+              #{association.foreign_key}: **value**
+
+            where **value** is the #{association.join_primary_key} value for the
+            associated #{association.klass.name} record.
+          MSG
+        end
+      end
+
       def initialize(fixture, table_rows:, label:, now:)
         @table_rows = table_rows
         @label = label
@@ -119,9 +142,13 @@ module ActiveRecord
               fk_name = association.join_foreign_key
 
               if association.name.to_s != fk_name && value = @row.delete(association.name.to_s)
-                if association.polymorphic? && value.sub!(/\s*\(([^)]*)\)\s*$/, "")
-                  # support polymorphic belongs_to as "label (Type)"
-                  @row[association.join_foreign_type] = $1
+                if association.polymorphic?
+                  if value.sub!(/\s*\(([^)]*)\)\s*$/, "")
+                    # support polymorphic belongs_to as "label (Type)"
+                    @row[association.join_foreign_type] = $1
+                  end
+                elsif association.join_primary_key != association.klass.primary_key
+                  raise PrimaryKeyError.new(@label, association, value)
                 end
 
                 fk_type = reflection_class.type_for_attribute(fk_name).type

activerecord/test/cases/fixtures_test.rb

@@ -6,6 +6,7 @@ require "models/admin"
 require "models/admin/account"
 require "models/admin/randomly_named_c1"
 require "models/admin/user"
+require "models/author"
 require "models/binary"
 require "models/book"
 require "models/bulb"
@@ -18,6 +19,7 @@ require "models/course"
 require "models/developer"
 require "models/dog"
 require "models/doubloon"
+require "models/essay"
 require "models/joke"
 require "models/matey"
 require "models/other_dog"
@@ -1492,6 +1494,16 @@ class FileFixtureConflictTest < ActiveRecord::TestCase
   end
 end
 
+class PrimaryKeyErrorTest < ActiveRecord::TestCase
+  test "generates the correct value" do
+    e = assert_raise(ActiveRecord::FixtureSet::TableRow::PrimaryKeyError) do
+      ActiveRecord::FixtureSet.create_fixtures(FIXTURES_ROOT + "/primary_key_error", "primary_key_error")
+    end
+
+    assert_includes e.message, "Unable to set"
+  end
+end
+
 if current_adapter?(:SQLite3Adapter) && !in_memory_db?
   class MultipleFixtureConnectionsTest < ActiveRecord::TestCase
     include ActiveRecord::TestFixtures

activerecord/test/fixtures/primary_key_error/primary_key_error.yml

@@ -0,0 +1,6 @@
+_fixture:
+  model_class: Author
+
+david:
+  name: David
+  owned_essay: a_modest_proposal