diff --git a/test/test_orm_has_one_through.rb b/test/test_orm_has_one_through.rb
new file mode 100644
index 0000000..e9cfc55
--- /dev/null
+++ b/test/test_orm_has_one_through.rb
@@ -0,0 +1,178 @@
+require 'minitest/autorun'
+require 'prism'
+require 'tmpdir'
+
+class OrmHasOneThroughTest < Minitest::Test
+  def setup
+    @dir = Dir.mktmpdir('prism-has-one-through-store')
+    @store = CountingHasOneThroughStore.new(File.join(@dir, 'test.pstore'))
+    @store.create_table(:suppliers, %w[id name])
+    @store.create_table(:accounts, %w[id supplier_id plan])
+    @store.create_table(:account_histories, %w[id account_id notes])
+
+    supplier_class = Class.new(Prism::Orm::Model) do
+      table_name :suppliers
+      has_one :account, class_name: 'Account', foreign_key: 'supplier_id'
+      has_one :account_history, through: :account
+    end
+    account_class = Class.new(Prism::Orm::Model) do
+      table_name :accounts
+      belongs_to :supplier, class_name: 'Supplier', foreign_key: 'supplier_id'
+      has_one :account_history, class_name: 'AccountHistory', foreign_key: 'account_id'
+    end
+    history_class = Class.new(Prism::Orm::Model) do
+      table_name :account_histories
+      belongs_to :account, class_name: 'Account', foreign_key: 'account_id'
+      has_one :supplier, through: :account
+    end
+    [supplier_class, account_class, history_class].each { |klass| klass.store = @store }
+    Object.const_set(:Supplier, supplier_class)
+    Object.const_set(:Account, account_class)
+    Object.const_set(:AccountHistory, history_class)
+  end
+
+  def teardown
+    FileUtils.remove_entry(@dir)
+    %i[Supplier Account AccountHistory].each { |name| Object.send(:remove_const, name) if Object.const_defined?(name) }
+  end
+
+  def test_has_one_through_reads_source_off_a_has_one_through_record
+    supplier = Supplier.create!(name: 'acme')
+    account = Account.create!(supplier_id: supplier.id, plan: 'gold')
+    history = AccountHistory.create!(account_id: account.id, notes: 'upgraded')
+
+    assert_equal history.id, supplier.account_history.id
+  end
+
+  def test_has_one_through_reads_source_off_a_belongs_to_through_record
+    supplier = Supplier.create!(name: 'acme')
+    account = Account.create!(supplier_id: supplier.id, plan: 'gold')
+    history = AccountHistory.create!(account_id: account.id, notes: 'upgraded')
+
+    assert_equal supplier.id, history.supplier.id
+  end
+
+  def test_has_one_through_returns_nil_without_through_record
+    supplier = Supplier.create!(name: 'lonely')
+    assert_nil supplier.account_history
+  end
+
+  def test_has_one_through_returns_nil_when_through_exists_but_source_missing
+    supplier = Supplier.create!(name: 'acme')
+    Account.create!(supplier_id: supplier.id, plan: 'gold')
+    assert_nil supplier.account_history
+  end
+
+  def test_has_one_through_honors_explicit_source_when_name_differs
+    history_with_alias = Class.new(Prism::Orm::Model) do
+      table_name :account_histories
+      belongs_to :account, class_name: 'Account', foreign_key: 'account_id'
+      has_one :owner, through: :account, source: :supplier
+    end
+    history_with_alias.store = @store
+
+    supplier = Supplier.create!(name: 'acme')
+    account = Account.create!(supplier_id: supplier.id, plan: 'gold')
+    record = history_with_alias.create!(account_id: account.id, notes: 'n/a')
+
+    assert_equal supplier.id, record.owner.id
+  end
+
+  def test_has_one_through_rejects_dependent_option
+    error = assert_raises(ArgumentError) do
+      Class.new(Prism::Orm::Model) do
+        table_name :suppliers
+        has_one :account, class_name: 'Account', foreign_key: 'supplier_id'
+        has_one :account_history, through: :account, dependent: :destroy
+      end
+    end
+    assert_match(/dependent/, error.message)
+  end
+
+  def test_includes_has_one_through_avoids_n_plus_one_queries
+    3.times do |i|
+      supplier = Supplier.create!(name: "supplier#{i}")
+      account = Account.create!(supplier_id: supplier.id, plan: 'gold')
+      AccountHistory.create!(account_id: account.id, notes: "notes#{i}")
+    end
+    lonely = Supplier.create!(name: 'no account')
+
+    @store.reset_query_count!
+    suppliers = Supplier.all.includes(:account_history).to_a
+    baseline = @store.query_count
+
+    notes = suppliers.map { |supplier| supplier.account_history&.notes }
+    assert_equal baseline, @store.query_count,
+      "expected preloaded #account_history to be free, but it issued #{@store.query_count - baseline} more queries"
+    assert_equal ['notes0', 'notes1', 'notes2', nil], notes.compact.sort + [nil]
+    assert_includes suppliers.map(&:name), lonely.name
+  end
+
+  # Mirrors test_includes_has_one_through_avoids_n_plus_one_queries above,
+  # but from the AccountHistory side where the through step (:account) is a
+  # belongs_to rather than a has_one - exercises the `when :belongs_to then
+  # preload_belongs_to(...)` branch in preload_has_one_through specifically,
+  # which the has_one-through variant above never touches.
+  def test_includes_has_one_through_by_belongs_to_avoids_n_plus_one_queries
+    3.times do |i|
+      supplier = Supplier.create!(name: "supplier#{i}")
+      account = Account.create!(supplier_id: supplier.id, plan: 'gold')
+      AccountHistory.create!(account_id: account.id, notes: "notes#{i}")
+    end
+
+    @store.reset_query_count!
+    histories = AccountHistory.all.includes(:supplier).to_a
+    baseline = @store.query_count
+
+    names = histories.map { |history| history.supplier.name }.sort
+    assert_equal baseline, @store.query_count,
+      "expected preloaded #supplier to be free, but it issued #{@store.query_count - baseline} more queries"
+    assert_equal %w[supplier0 supplier1 supplier2], names
+  end
+
+  def test_includes_has_one_through_unknown_through_raises
+    broken_class = Class.new(Prism::Orm::Model) do
+      table_name :suppliers
+      has_one :account_history, through: :nonexistent
+    end
+    broken_class.store = @store
+    broken_class.create!(name: 'x')
+    assert_raises(ArgumentError) { broken_class.all.includes(:account_history).to_a }
+  end
+
+  def test_includes_has_one_through_rejects_has_many_through_association
+    broken_class = Class.new(Prism::Orm::Model) do
+      table_name :suppliers
+      has_many :accounts, class_name: 'Account', foreign_key: 'supplier_id'
+      has_one :account_history, through: :accounts
+    end
+    broken_class.store = @store
+    broken_class.create!(name: 'x')
+    assert_raises(ArgumentError) { broken_class.all.includes(:account_history).to_a }
+  end
+end
+
+# Wraps Store#all/#find to count reads, mirroring CountingThroughStore in
+# test_orm_has_many_through.rb so this file has no load-order dependency.
+class CountingHasOneThroughStore < Prism::Orm::Store
+  def initialize(*)
+    super
+    @query_count = 0
+  end
+
+  attr_reader :query_count
+
+  def reset_query_count!
+    @query_count = 0
+  end
+
+  def all(table)
+    @query_count += 1
+    super
+  end
+
+  def find(table, id)
+    @query_count += 1
+    super
+  end
+end