A reflection on filters

[remyduthu]

Hi Ben, first of all thank you for your articles which helped me a lot to structure my Go projects! I'd like to share a function that uses the reflect package to construct a WHERE clause from a filter structure. Here is the function:

// where builds a WHERE SQL clause based on given filter. Use `column:""` field
// tag in the filter to add it to the clause. The value of the tag should match
// the name of the column in the database (e.g. `column:"user_id"`).
func where(f interface{}) string {
	// Start with an always true predicate. Thus, "SELECT * FROM x WHERE 1 = 1" is
	// equivalent to "SELECT * FROM x".
	where := "1 = 1"

	v := reflect.ValueOf(f)

	// Follow pointers to work with underlying element
	for v.Kind() == reflect.Ptr {
		v = v.Elem()
	}

	// This function only works with structures
	if v.Kind() != reflect.Struct {
		return where
	}

	// Browse fields
	for i := 0; i < v.NumField(); i++ {
		field := v.Field(i)

		// Skip missing fields
		if field.IsZero() {
			continue
		}

		if tag := v.Type().Field(i).Tag.Get("column"); tag != "" {
			// Complete clause
			where = fmt.Sprintf(
				`%s AND %s = '%s'`, where, tag, field.String(),
			)
		}
	}

	return where
}

And, here is an example of a filter:

type UserFilter struct {
	Name   string `column:"name"`
	Email  string `column:"email"`
	Limit  int
	Offset int
}

The function will dynamically build the clause using the Name and Email fields.

[benbjohnson]

Hi @remyduthu. Thanks for posting. I tend to be hesitant to use reflection in Go most of the type. I find it's easier to introduce bugs personally. It's a cool implementation though. :)