diff --git a/test/test_orm_pessimistic.rb b/test/test_orm_pessimistic.rb
new file mode 100644
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--- /dev/null
+++ b/test/test_orm_pessimistic.rb
@@ -0,0 +1,161 @@
+require 'minitest/autorun'
+require 'prism'
+require 'tmpdir'
+require 'fileutils'
+
+class OrmPessimisticTest < Minitest::Test
+  def setup
+    @dir = Dir.mktmpdir('prism-pessimistic')
+    @store = Prism::Orm::Store.new(File.join(@dir, 'test.pstore'))
+    @store.create_table(:counters, %w[id value])
+
+    counter_class = Class.new(Prism::Orm::Model) { table_name :counters }
+    counter_class.store = @store
+    Object.const_set(:Counter, counter_class)
+  end
+
+  def teardown
+    FileUtils.remove_entry(@dir)
+    Object.send(:remove_const, :Counter) if Object.const_defined?(:Counter)
+  end
+
+  def test_lock_bang_raises_on_a_record_that_was_never_saved
+    counter = Counter.new(value: 0)
+    assert_raises(Prism::Orm::Pessimistic::UnpersistedRecordError) { counter.lock! }
+  end
+
+  def test_lock_bang_raises_on_a_record_with_unsaved_changes
+    counter = Counter.create!(value: 0)
+    counter.value = 5
+    assert_raises(Prism::Orm::Pessimistic::DirtyRecordError) { counter.lock! }
+  end
+
+  def test_lock_bang_reloads_data_written_by_another_handle_in_the_meantime
+    counter = Counter.create!(value: 0)
+    Counter.find(counter.id).update!(value: 41)
+
+    counter.lock!
+
+    assert_equal 41, counter.value
+  end
+
+  def test_lock_bang_returns_self
+    counter = Counter.create!(value: 0)
+    assert_same counter, counter.lock!
+  end
+
+  def test_with_lock_yields_the_record_with_freshly_reloaded_data
+    counter = Counter.create!(value: 0)
+    Counter.find(counter.id).update!(value: 9)
+
+    yielded = nil
+    counter.with_lock { |c| yielded = c }
+
+    assert_same counter, yielded
+    assert_equal 9, counter.value
+  end
+
+  def test_with_lock_commits_a_write_made_inside_the_block
+    counter = Counter.create!(value: 1)
+    counter.with_lock { |c| c.update!(value: c.value + 1) }
+    assert_equal 2, Counter.find(counter.id).value
+  end
+
+  def test_with_lock_rolls_back_a_write_made_inside_the_block_on_exception
+    counter = Counter.create!(value: 1)
+    assert_raises(RuntimeError) do
+      counter.with_lock do |c|
+        c.update!(value: 99)
+        raise 'boom'
+      end
+    end
+    assert_equal 1, Counter.find(counter.id).value
+  end
+
+  def test_with_lock_nests_inside_an_already_open_transaction_without_deadlocking
+    counter = Counter.create!(value: 1)
+    Counter.transaction do
+      counter.with_lock { |c| c.update!(value: c.value + 1) }
+    end
+    assert_equal 2, Counter.find(counter.id).value
+  end
+
+  # Everything below forks real child OS processes sharing nothing but the
+  # PStore file on disk - Store#within_transaction opens a brand new file
+  # handle and takes a real File::LOCK_EX/LOCK_SH on it for every single
+  # transaction (stdlib pstore.rb, not something this codebase controls),
+  # so this is the same cross-process guarantee two independent `prism`
+  # server processes pointed at the same database file would get, not a
+  # simulation of it via threads. `exit!` (not `exit`) is required in every
+  # forked child: minitest/autorun installs an at_exit hook, and a plain
+  # `exit` would let the child fall into that hook and re-run the entire
+  # suite a second time from inside the fork.
+  def test_two_processes_updating_without_a_lock_lose_an_update
+    skip 'fork not supported on this platform' unless Process.respond_to?(:fork)
+
+    counter = Counter.create!(value: 0)
+    a_read, a_wrote_read_signal = IO.pipe
+    b_read, b_wrote_read_signal = IO.pipe
+
+    # Two pipes force the full ordering a lost update needs: A reads, THEN
+    # B reads (while the value is still whatever A saw, because A is
+    # blocked waiting on B before it writes anything), THEN both write back
+    # "their own read + 1" as two independent transactions. Only the READS
+    # are ordered here - deliberately NOT the writes, since a real lost
+    # update does not care which write lands last, only that both were
+    # computed from the same stale value.
+    pid_a = Process.fork do
+      a_read.close
+      b_wrote_read_signal.close
+      value = Counter.find(counter.id).value
+      a_wrote_read_signal.write('r')
+      a_wrote_read_signal.close
+      b_read.read(1)
+      b_read.close
+      Counter.find(counter.id).update!(value: value + 1)
+      exit!(0)
+    end
+
+    pid_b = Process.fork do
+      a_wrote_read_signal.close
+      b_read.close
+      a_read.read(1)
+      a_read.close
+      value = Counter.find(counter.id).value
+      b_wrote_read_signal.write('r')
+      b_wrote_read_signal.close
+      Counter.find(counter.id).update!(value: value + 1)
+      exit!(0)
+    end
+
+    [a_read, a_wrote_read_signal, b_read, b_wrote_read_signal].each(&:close)
+    Process.waitpid(pid_a)
+    Process.waitpid(pid_b)
+
+    # Both processes read 0 (forced ordering above only sequences the
+    # reads), so both independently write 1 back - the second write just
+    # clobbers the first with the exact same value, silently losing A's
+    # increment no matter which of the two writes lands last.
+    assert_equal 1, Counter.find(counter.id).value
+  end
+
+  def test_two_processes_incrementing_through_with_lock_never_lose_an_update
+    skip 'fork not supported on this platform' unless Process.respond_to?(:fork)
+
+    counter = Counter.create!(value: 0)
+    increments_per_process = 5
+
+    pids = Array.new(2) do
+      Process.fork do
+        increments_per_process.times do
+          Counter.find(counter.id).with_lock { |c| c.update!(value: c.value + 1) }
+        end
+        exit!(0)
+      end
+    end
+
+    pids.each { |pid| Process.waitpid(pid) }
+
+    assert_equal 2 * increments_per_process, Counter.find(counter.id).value
+  end
+end