diff --git a/docs/reference.md b/docs/reference.md
index 2a9b6c1..d9eb4fd 100644
--- a/docs/reference.md
+++ b/docs/reference.md
@@ -78,6 +78,36 @@ end
 `Definition#expose(**records)`:
 - Exposed names become repository methods.
 - 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.
+
+Because capture happens when `expose` is called, expose a record only once it
+has been saved:
+
+```ruby
+FixtureKit.define do
+  user = User.new(name: "Alice")
+  expose(user: user)  # raises FixtureKit::UnpersistedRecordError
+  user.save!
+end
+```
+
+The same timing applies to collections, but an emptied or later-appended
+collection cannot be detected — it is captured as-is:
+
+```ruby
+FixtureKit.define do
+  projects = []
+  expose(projects: projects)          # captured as an empty collection
+  projects << Project.create!(...)    # not reflected in `fixture.projects`
+end
+```
+
+The conventional form -- `expose` as the last statement of the definition --
+avoids both cases.
 
 ## Fixture Inheritance (`extends`)
 
@@ -337,6 +367,7 @@ Public error classes:
 - `FixtureKit::FixtureDefinitionNotFound`
 - `FixtureKit::RunnerAlreadyStartedError`
 - `FixtureKit::CircularFixtureInheritance`
+- `FixtureKit::UnpersistedRecordError`
 
 ## Requirements
 
diff --git a/lib/fixture_kit.rb b/lib/fixture_kit.rb
index 7caf094..a718648 100644
--- a/lib/fixture_kit.rb
+++ b/lib/fixture_kit.rb
@@ -9,6 +9,7 @@ class CacheMissingError < Error; end
   class FixtureDefinitionNotFound < Error; end
   class RunnerAlreadyStartedError < Error; end
   class CircularFixtureInheritance < Error; end
+  class UnpersistedRecordError < Error; end
 
   autoload :VERSION, File.expand_path("fixture_kit/version", __dir__)
   autoload :Configuration, File.expand_path("fixture_kit/configuration", __dir__)
diff --git a/lib/fixture_kit/cache.rb b/lib/fixture_kit/cache.rb
index c31efba..34ba0c9 100644
--- a/lib/fixture_kit/cache.rb
+++ b/lib/fixture_kit/cache.rb
@@ -53,7 +53,7 @@ def save
       FixtureKit.runner.adapter.execute do |context|
         @content = MemoryCache.new(
           data: evaluate(FixtureKit.runner.coders, context),
-          exposed: file_cache.serialize_exposed(fixture.definition.exposed)
+          exposed: fixture.definition.exposed
         )
       end
 
diff --git a/lib/fixture_kit/definition.rb b/lib/fixture_kit/definition.rb
index 4939892..e646551 100644
--- a/lib/fixture_kit/definition.rb
+++ b/lib/fixture_kit/definition.rb
@@ -25,12 +25,31 @@ def expose(**records)
           raise FixtureKit::DuplicateNameError, "Name #{name} already exposed"
         end
 
-        @exposed[name] = record
+        @exposed[name] = serialize(name, record)
       end
     end
 
     private
 
+    def serialize(name, record)
+      if record.is_a?(Array)
+        record.map { |item| reference(name, item) }
+      else
+        reference(name, record)
+      end
+    end
+
+    def reference(name, record)
+      unless record.persisted?
+        raise FixtureKit::UnpersistedRecordError,
+          "cannot expose #{name.inspect}: the #{record.class} is not persisted. " \
+          "Exposed records are captured as class/id pairs when `expose` is called, " \
+          "so save the record before exposing it."
+      end
+
+      { record.class => record.id }
+    end
+
     def mixin(parent)
       definition = self
 
diff --git a/lib/fixture_kit/file_cache.rb b/lib/fixture_kit/file_cache.rb
index 51da248..e9f7fe5 100644
--- a/lib/fixture_kit/file_cache.rb
+++ b/lib/fixture_kit/file_cache.rb
@@ -48,16 +48,6 @@ def write(data)
       File.write(path, JSON.pretty_generate(content))
     end
 
-    def serialize_exposed(exposed)
-      exposed.each_with_object({}) do |(name, record), hash|
-        if record.is_a?(Array)
-          hash[name] = record.map { |record| { record.class => record.id } }
-        else
-          hash[name] = { record.class => record.id }
-        end
-      end
-    end
-
     private
 
     def coder_for(class_name)
diff --git a/spec/unit/definition_spec.rb b/spec/unit/definition_spec.rb
index a2eccbc..58534d0 100644
--- a/spec/unit/definition_spec.rb
+++ b/spec/unit/definition_spec.rb
@@ -12,41 +12,43 @@
   end
 
   describe "#evaluate" do
+    let(:alice) { User.create!(name: "Alice", email: "alice-definition@example.com") }
+
     it "captures exposed records from the definition block" do
+      record = alice
       definition = described_class.new do
-        expose(alice: "alice")
+        expose(alice: record)
       end
 
       definition.evaluate(Object.new)
 
-      expect(definition.exposed).to eq({ alice: "alice" })
+      expect(definition.exposed).to eq({ alice: { User => alice.id } })
     end
 
     it "supports helper methods from execution context" do
+      record = alice
       helper_context = Class.new do
-        def fixture_name
-          "alice"
-        end
+        define_method(:fixture_record) { record }
       end.new
 
       definition = described_class.new do
-        expose(alice: fixture_name)
+        expose(alice: fixture_record)
       end
 
       definition.evaluate(helper_context)
 
-      expect(definition.exposed).to eq({ alice: "alice" })
+      expect(definition.exposed).to eq({ alice: { User => alice.id } })
     end
 
     it "supports parent helper in execution context" do
-      parent_repository = Struct.new(:owner).new("alice")
+      parent_repository = Struct.new(:owner).new(alice)
       definition = described_class.new do
         expose(owner: parent.owner)
       end
 
       definition.evaluate(Object.new, parent: parent_repository)
 
-      expect(definition.exposed).to eq({ owner: "alice" })
+      expect(definition.exposed).to eq({ owner: { User => alice.id } })
     end
   end
 
@@ -60,14 +62,94 @@ def fixture_name
 
   describe "#expose" do
     it "raises when the same name is exposed twice" do
+      alice = User.create!(name: "Alice", email: "alice-duplicate@example.com")
       definition = described_class.new do
-        expose(alice: "alice")
-        expose(alice: "duplicate")
+        expose(alice: alice)
+        expose(alice: alice)
       end
 
       expect do
         definition.evaluate(Object.new)
       end.to raise_error(FixtureKit::DuplicateNameError, "Name alice already exposed")
     end
+
+    it "stores a record as a class/id pair" do
+      alice = User.create!(name: "Alice", email: "alice-pair@example.com")
+      definition = described_class.new { expose(alice: alice) }
+
+      definition.evaluate(Object.new)
+
+      expect(definition.exposed).to eq({ alice: { User => alice.id } })
+    end
+
+    it "stores a collection as an array of class/id pairs" do
+      alice = User.create!(name: "Alice", email: "alice-collection@example.com")
+      bob = User.create!(name: "Bob", email: "bob-collection@example.com")
+      definition = described_class.new { expose(users: [alice, bob]) }
+
+      definition.evaluate(Object.new)
+
+      expect(definition.exposed).to eq({ users: [{ User => alice.id }, { User => bob.id }] })
+    end
+
+    it "stores the record's own class for single-table-inheritance records" do
+      sedan = Car.create!(name: "Sedan")
+      definition = described_class.new { expose(sedan: sedan) }
+
+      definition.evaluate(Object.new)
+
+      expect(definition.exposed).to eq({ sedan: { Car => sedan.id } })
+    end
+
+    it "raises when the record is not persisted" 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/
+      )
+    end
+
+    it "raises when a record inside a collection is not persisted" 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]) }
+
+      expect { definition.evaluate(Object.new) }.to raise_error(
+        FixtureKit::UnpersistedRecordError,
+        /cannot expose :users/
+      )
+    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) }
+
+      expect { definition.evaluate(Object.new) }
+        .to raise_error(FixtureKit::UnpersistedRecordError)
+    end
+
+    it "allows an empty collection" do
+      definition = described_class.new { expose(users: []) }
+
+      definition.evaluate(Object.new)
+
+      expect(definition.exposed).to eq({ users: [] })
+    end
+
+    # The registry holds every fixture for the life of the process, so a
+    # definition that held its records would keep their whole object graph
+    # alive until the suite ends.
+    it "does not hold a reference to the exposed record" do
+      alice = User.create!(name: "Alice", email: "alice-noref@example.com")
+      definition = described_class.new { expose(alice: alice) }
+
+      definition.evaluate(Object.new)
+
+      expect(definition.exposed.values.flatten).to all(be_a(Hash))
+      expect(definition.exposed.values.flatten.flat_map(&:values)).to all(be_a(Integer))
+    end
   end
 end
diff --git a/spec/unit/file_cache_spec.rb b/spec/unit/file_cache_spec.rb
index e3415e8..77d4c2a 100644
--- a/spec/unit/file_cache_spec.rb
+++ b/spec/unit/file_cache_spec.rb
@@ -84,23 +84,4 @@
       expect(File.exist?(nested_path)).to be(true)
     end
   end
-
-  describe "#serialize_exposed" do
-    it "converts ActiveRecord instances to class/id pairs" do
-      user = User.create!(name: "Alice", email: "alice-file-cache@example.com")
-
-      result = file_cache.serialize_exposed({ alice: user })
-
-      expect(result).to eq({ alice: { User => user.id } })
-    end
-
-    it "converts arrays of ActiveRecord instances" do
-      alice = User.create!(name: "Alice", email: "alice-array@example.com")
-      bob = User.create!(name: "Bob", email: "bob-array@example.com")
-
-      result = file_cache.serialize_exposed({ users: [alice, bob] })
-
-      expect(result).to eq({ users: [{ User => alice.id }, { User => bob.id }] })
-    end
-  end
 end
diff --git a/spec/unit/fixture_cache_spec.rb b/spec/unit/fixture_cache_spec.rb
index 8d4a1bf..0926397 100644
--- a/spec/unit/fixture_cache_spec.rb
+++ b/spec/unit/fixture_cache_spec.rb
@@ -226,6 +226,29 @@ def identifier_for(identifier)
       expect(User.count).to eq(0)
     end
 
+    # The registry holds every fixture for the life of the process, so a
+    # definition that kept its exposed records would keep their whole object
+    # graph alive until the suite ends.
+    it "caches exposed records as class/id pairs without retaining the records" do
+      fixture_definition = FixtureKit::Definition.new do
+        alice = User.create!(name: "Alice", email: "alice-release@example.com")
+        expose(alice: alice)
+      end
+      fixture_double = instance_double(
+        FixtureKit::Fixture,
+        identifier: fixture_name,
+        definition: fixture_definition,
+        parent: nil
+      )
+      fixture_cache = described_class.new(fixture_double)
+
+      fixture_cache.save
+
+      expect(fixture_definition.exposed.fetch(:alice).keys).to eq([User])
+      expect(fixture_definition.exposed.fetch(:alice).values.first).to be_a(Integer)
+      expect(fixture_cache.load.alice.name).to eq("Alice")
+    end
+
     it "includes parent fixture model records when saving inherited fixtures" do
       parent_cache_data = FixtureKit::MemoryCache.new(
         data: { FixtureKit::ActiveRecordCoder => { User => nil } },