Implement ApplyIf for UpdateBuilder

Fixes #6

builder/update_builder.go

@@ -196,3 +196,13 @@ func (b UpdateBuilder) innerWriteSQL(sb *SQLBuilder) {
 		b.returningItems.WriteSQL(sb)
 	}
 }
+
+// ApplyIf applies the given function to the builder if the condition is true.
+// It returns the builder itself if the condition is false, otherwise it returns the result of the function.
+// It's especially helpful for building a query conditionally.
+func (b UpdateBuilder) ApplyIf(cond bool, apply func(q UpdateBuilder) UpdateBuilder) UpdateBuilder {
+	if cond && apply != nil {
+		return apply(b)
+	}
+	return b
+}

update_builder_test.go

@@ -4,6 +4,7 @@ import (
 	"testing"
 
 	"github.com/networkteam/qrb"
+	"github.com/networkteam/qrb/builder"
 	"github.com/networkteam/qrb/internal/testhelper"
 )
 
@@ -133,4 +134,23 @@ func TestUpdateBuilder(t *testing.T) {
 			q,
 		)
 	})
+
+	t.Run("apply if", func(t *testing.T) {
+		q := qrb.
+			Update(qrb.N("films")).
+			Set("kind", qrb.String("Dramatic")).
+			Where(qrb.N("kind").Eq(qrb.String("Drama"))).
+			ApplyIf(true, func(q builder.UpdateBuilder) builder.UpdateBuilder {
+				return q.Where(qrb.N("archived").Eq(qrb.Bool(false)))
+			})
+
+		testhelper.AssertSQLWriterEquals(
+			t,
+			`
+			UPDATE films SET kind = 'Dramatic' WHERE kind = 'Drama' AND archived = false
+			`,
+			nil,
+			q,
+		)
+	})
 }