Require an id when exposing a record, not persistence

docs/reference.md

@@ -80,9 +80,15 @@ end
 - Duplicate exposed names raise `FixtureKit::DuplicateNameError`.
 - Records are captured as class/id pairs at the moment `expose` is called, not
   at the end of the definition. The record objects themselves are not retained.
-- Exposing a record that is not persisted raises
-  `FixtureKit::UnpersistedRecordError`. This applies to records inside an
-  exposed collection as well, and to records that have been destroyed.
+- Exposing a record with no id raises `FixtureKit::UnpersistedRecordError`.
+  This applies to records inside an exposed collection as well.
+- Only the class and the id are stored, so an unsaved instance carrying the id
+  of a real row is a valid way to expose that row under a different model class
+  than the one that created it:
+
+  ```ruby
+  expose(address: Addresses::Db::Address.new(id: company_address.id))
+  ```
 
 Because capture happens when `expose` is called, expose a record only once it
 has been saved:

lib/fixture_kit/definition.rb

@@ -40,9 +40,9 @@ def serialize(name, record)
     end
 
     def reference(name, record)
-      unless record.persisted?
+      if record.id.nil?
         raise FixtureKit::UnpersistedRecordError,
-          "cannot expose #{name.inspect}: the #{record.class} is not persisted. " \
+          "cannot expose #{name.inspect}: the #{record.class} has no id. " \
           "Exposed records are captured as class/id pairs when `expose` is called, " \
           "so save the record before exposing it."
       end

spec/unit/definition_spec.rb

@@ -101,17 +101,17 @@
       expect(definition.exposed).to eq({ sedan: { Car => sedan.id } })
     end
 
-    it "raises when the record is not persisted" do
+    it "raises when the record has no id" do
       unsaved = User.new(name: "Alice", email: "alice-unsaved@example.com")
       definition = described_class.new { expose(alice: unsaved) }
 
       expect { definition.evaluate(Object.new) }.to raise_error(
         FixtureKit::UnpersistedRecordError,
-        /cannot expose :alice: the User is not persisted/
+        /cannot expose :alice: the User has no id/
       )
     end
 
-    it "raises when a record inside a collection is not persisted" do
+    it "raises when a record inside a collection has no id" do
       saved = User.create!(name: "Alice", email: "alice-mixed@example.com")
       unsaved = User.new(name: "Bob", email: "bob-mixed@example.com")
       definition = described_class.new { expose(users: [saved, unsaved]) }
@@ -122,13 +122,16 @@
       )
     end
 
-    it "raises when the record has been destroyed" do
-      destroyed = User.create!(name: "Alice", email: "alice-destroyed@example.com")
-      destroyed.destroy!
-      definition = described_class.new { expose(alice: destroyed) }
+    # Only the class and the id are stored, so an unsaved instance carrying the
+    # id of a real row is a valid way to expose that row under a different
+    # model class than the one that created it.
+    it "accepts an unsaved instance that carries the id of a real row" do
+      alice = User.create!(name: "Alice", email: "alice-reference@example.com")
+      definition = described_class.new { expose(alice: User.new(id: alice.id)) }
 
-      expect { definition.evaluate(Object.new) }
-        .to raise_error(FixtureKit::UnpersistedRecordError)
+      definition.evaluate(Object.new)
+
+      expect(definition.exposed).to eq({ alice: { User => alice.id } })
     end
 
     it "allows an empty collection" do