#977: Reject empty if_absent matching criteria

lib/fbe/if_absent.rb

@@ -65,7 +65,9 @@ def Fbe.if_absent(fb: Fbe.fb, always: false)
       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 if_absent') 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_if_absent.rb

@@ -49,6 +49,25 @@ def test_ignores
     assert_nil(n)
   end
 
+  def test_raises_when_block_sets_no_attributes
+    fb = Factbase.new
+    fb.insert.foo = 'unrelated'
+    error = assert_raises(Fbe::Error) do
+      Fbe.if_absent(fb:) { |_fact| }
+    end
+    assert_equal('At least one non-system attribute is required by if_absent', error.message)
+    assert_equal(1, fb.size)
+  end
+
+  def test_raises_when_only_system_attributes_are_set
+    fb = Factbase.new
+    error = assert_raises(Fbe::Error) do
+      Fbe.if_absent(fb:) { |fact| fact._id = 42 }
+    end
+    assert_equal('At least one non-system attribute is required by if_absent', error.message)
+    assert_equal(0, fb.size)
+  end
+
   def test_raises_on_empty_value
     assert_raises(StandardError) do
       Fbe.if_absent(fb: Factbase.new) do |f|