Set converter defaults once in Convert method

This will make the converter also work as expected when the
filter.Converter struct is created without the NewConverter function.

Author: koenbollen

filter/converter.go

@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"sort"
 	"strings"
+	"sync"
 )
 
 var basicOperatorMap = map[string]string{
@@ -33,23 +34,22 @@ type Converter struct {
 	}
 	emptyCondition  string
 	placeholderName string
+
+	once sync.Once
 }
 
 // NewConverter creates a new Converter with optional nested JSONB field mapping.
 //
 // Note: When using github.com/lib/pq, the filter.WithArrayDriver should be set to pq.Array.
 func NewConverter(options ...Option) *Converter {
 	converter := &Converter{
-		emptyCondition: "FALSE",
+		// don't set defaults, use the once.Do in #Convert()
 	}
 	for _, option := range options {
 		if option != nil {
 			option(converter)
 		}
 	}
-	if converter.placeholderName == "" {
-		converter.placeholderName = DefaultPlaceholderName
-	}
 	return converter
 }
 
@@ -58,6 +58,15 @@ func NewConverter(options ...Option) *Converter {
 // startAtParameterIndex is the index to start the parameter numbering at.
 // Passing X will make the first indexed parameter $X, the second $X+1, and so on.
 func (c *Converter) Convert(query []byte, startAtParameterIndex int) (conditions string, values []any, err error) {
+	c.once.Do(func() {
+		if c.emptyCondition == "" {
+			c.emptyCondition = "FALSE"
+		}
+		if c.placeholderName == "" {
+			c.placeholderName = DefaultPlaceholderName
+		}
+	})
+
 	if startAtParameterIndex < 1 {
 		return "", nil, fmt.Errorf("startAtParameterIndex must be greater than 0")
 	}

filter/converter_test.go

@@ -402,3 +402,28 @@ func TestConverter_WithEmptyCondition(t *testing.T) {
 		t.Errorf("Converter.Convert() values = %v, want nil", values)
 	}
 }
+
+func TestConverter_NoConstructor(t *testing.T) {
+	c := &filter.Converter{}
+	conditions, values, err := c.Convert([]byte(`{"name": "John"}`), 1)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if want := `("name" = $1)`; conditions != want {
+		t.Errorf("Converter.Convert() conditions = %v, want %v", conditions, want)
+	}
+	if !reflect.DeepEqual(values, []any{"John"}) {
+		t.Errorf("Converter.Convert() values = %v, want %v", values, []any{"John"})
+	}
+
+	conditions, values, err = c.Convert([]byte(``), 1)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if want := "FALSE"; conditions != want {
+		t.Errorf("Converter.Convert() conditions = %v, want %v", conditions, want)
+	}
+	if len(values) != 0 {
+		t.Errorf("Converter.Convert() values = %v, want nil", values)
+	}
+}