fix: avoid key to start with $ sign (#61)

filterparser.go

@@ -334,6 +334,10 @@ func (p *FilterParser) parseOperatorAndValue() (parserToken, interface{}, error)
 
 func (p *FilterParser) makeFilter(key string, operator parserToken, value interface{}) (*Filter, error) {
 
+	if strings.HasPrefix(key, "$") {
+		return nil, fmt.Errorf("could not start a parameter with $. Found %s", key)
+	}
+
 	filter := NewFilterComposer()
 
 	// Create filter

filterparser_test.go

@@ -235,16 +235,16 @@ func TestParser_Keys(t *testing.T) {
 	Convey("Given the expression is a tag like '$key'", t, func() {
 
 		parser := NewFilterParser("$key == value")
-		expectedFilter := NewFilterComposer().WithKey("$key").Equals("value").Done()
+		// expectedFilter := NewFilterComposer().WithKey("$key").Equals("value").Done()
 
 		Convey("When I run Parse", func() {
 
 			filter, err := parser.Parse()
 
-			Convey("Then there should be no error and the filter should as expected", func() {
-				So(err, ShouldEqual, nil)
-				So(filter, ShouldNotEqual, nil)
-				So(filter.String(), ShouldEqual, expectedFilter.String())
+			Convey("Then there should have an error", func() {
+				So(err, ShouldNotEqual, nil)
+				So(err.Error(), ShouldContainSubstring, "could not start a parameter with $")
+				So(filter, ShouldEqual, nil)
 			})
 		})
 	})