add a FieldList type

This type represents a list of fields someone might want to have
returned.
This can be used together with the query library to build select
statements that return the exact data the user might need or want.

This way we may be able to avoid selecting data one might not need at
the end and therefore provide better performance.

types/fields.go

@@ -0,0 +1,64 @@
+package types
+
+import (
+	"encoding/json"
+	"fmt"
+	"regexp"
+	"sort"
+)
+
+type (
+	// FieldList is a parameter type to represent a list of fields in the database
+	// to return. It can be used with query.FieldListToSelect to build a
+	// select clause to return the data that was requested.
+	FieldList struct {
+		fields map[string]bool
+	}
+)
+
+var (
+	// fieldIdentifier filters field names to allow only sane values
+	// and at the same time make them save for database queries.
+	fieldIdentifier = regexp.MustCompile(`\A[a-zA-Z]+[a-zA-Z0-9_\-]`)
+)
+
+func NewFieldList(fields ...string) FieldList {
+	fl := FieldList{
+		fields: map[string]bool{},
+	}
+	for _, field := range fields {
+		fl.fields[field] = true
+	}
+	return fl
+}
+
+func (fl *FieldList) UnmarshalJSON(raw []byte) error {
+	fields := []string{}
+	if err := json.Unmarshal(raw, &fields); err != nil {
+		return err
+	}
+	fl.fields = map[string]bool{}
+	for _, field := range fields {
+		if !fieldIdentifier.Match([]byte(field)) {
+			return fmt.Errorf("`%s` is not an allowed field name. Allowed is only alpha numerical", field)
+		}
+		fl.fields[field] = true
+	}
+	return nil
+}
+
+// Contains returns true, when the string is found in the list.
+func (fl FieldList) Contains(in string) bool {
+	_, found := fl.fields[in]
+	return found
+}
+
+// Fields returns a sorted list of the requested fields.
+func (fl FieldList) Fields() []string {
+	res := []string{}
+	for name, _ := range fl.fields {
+		res = append(res, name)
+	}
+	sort.Strings(res)
+	return res
+}