#854: Reject empty just_one matching criteria

lib/fbe/just_one.rb

@@ -50,7 +50,9 @@ def Fbe.just_one(fb: Fbe.fb)
       end
     end
   yield(f)
-  q = attrs.except(:_id, :_time, :_version).map do |k, v|
+  matching = attrs.except(:_id, :_time, :_version)
+  raise(Fbe::Error, 'At least one non-system attribute is required by just_one') if matching.empty?
+  q = matching.map do |k, v|
     raise(Fbe::Error, "Can't match #{k} by an array, only by one value") if v.is_a?(Array)
     vv = v.to_s
     if v.is_a?(String)

test/fbe/test_just_one.rb

@@ -73,6 +73,27 @@ def test_raises_on_nil
     end
   end
 
+  def test_rejects_empty_matching_attributes
+    fb = Factbase.new
+    fb.insert.foo = 'existing'
+    error = assert_raises(Fbe::Error) do
+      Fbe.just_one(fb:) { |_f| }
+    end
+    assert_match(/non-system attribute/, error.message)
+    assert_equal(1, fb.size)
+  end
+
+  def test_rejects_only_system_matching_attributes
+    fb = Factbase.new
+    error = assert_raises(Fbe::Error) do
+      Fbe.just_one(fb:) do |f|
+        f._id = 42
+      end
+    end
+    assert_match(/non-system attribute/, error.message)
+    assert_equal(0, fb.size)
+  end
+
   def test_raises_without_block
     assert_raises(Fbe::Error, 'just_one accepted a call without a block') do
       Fbe.just_one(fb: Factbase.new)