refs #78 add new fieldas tag to set AS name for SELECT

README.md

@@ -136,6 +136,7 @@ type ATable struct {
     Field2     int    `db:"field2"`                      // Use "db" in field tag to set column name used in SQL.
     Field3     int64  `db:"field3" fieldtag:"foo,bar"`   // Set fieldtag to a field. We can use methods like `Struct#SelectForTag` to use it.
     Field4     int64  `db:"field4" fieldtag:"foo"`       // If we use `s.SelectForTag(table, "foo")`, columnes of SELECT are field3 and field4.
+    Field5As  string `db:"field5" fieldas:"FIELD5"`      // Use "fieldas" in field tag to set AS name used in SELECT.
     Ignored    int32  `db:"-"`                           // If we set field name as "-", Struct will ignore it.
     unexported int                                       // Unexported field is not visible to Struct.
     Quoted     string `db:"quoted" fieldopt:"withquote"` // Add quote to the field using back quote or double quote. See `Flavor#Quote`.

struct.go

@@ -22,6 +22,9 @@ var (
 	// FieldOpt is the options for a struct field.
 	// As db column can contain "," in theory, field options should be provided in a separated tag.
 	FieldOpt = "fieldopt"
+
+	// FieldAs is the AS name for a struct field.
+	FieldAs = "fieldas"
 )
 
 const (
@@ -117,7 +120,13 @@ func (s *Struct) SelectFromForTag(table string, tag string) *SelectBuilder {
 		for _, field := range fields {
 			buf.WriteString(table)
 			buf.WriteRune('.')
-			buf.WriteString(field)
+
+			if fieldas, exists := sf.fieldAs[field]; exists {
+				buf.WriteString(sb.As(field, fieldas))
+			} else {
+				buf.WriteString(field)
+			}
+
 			cols = append(cols, buf.String())
 			buf.Reset()
 		}

struct_test.go

@@ -776,6 +776,22 @@ func TestStructFieldMapper(t *testing.T) {
 	a.Equal(sql, "SELECT t.`FieldName1`, t.set_by_tag, t.field_name1, t.EmbeddedField2, t.EmbeddedAndEmbeddedField1 FROM t")
 }
 
+type structWithAs struct {
+	T1 string `db:"t1" fieldas:"f1"`
+	T2 string `db:"t2" fieldas:""`   // Empty fieldas is the same as the tag is not set.
+	T3 string `fieldas:"f3"`         // AS works without db tag.
+	T4 string `db:"t4" fieldas:"f3"` // fieldas tag can duplicate.
+}
+
+func TestStructFieldAs(t *testing.T) {
+	a := assert.New(t)
+	s := NewStruct(new(structWithAs))
+	sb := s.SelectFrom("t")
+	b := Build(`COPY ($?) TO '/path/to/file.csv' (FORMAT CSV, HEADER)`, sb)
+	sql, _ := b.Build()
+	a.Equal(sql, `COPY (SELECT t.t1 AS f1, t.t2, t.T3 AS f3, t.t4 AS f3 FROM t) TO '/path/to/file.csv' (FORMAT CSV, HEADER)`)
+}
+
 func SomeOtherMapper(string) string {
 	return ""
 }

structfields.go

@@ -7,6 +7,7 @@ import (
 
 type structFields struct {
 	fieldAlias      map[string]string
+	fieldAs         map[string]string
 	taggedFields    map[string][]string
 	quotedFields    map[string]struct{}
 	omitEmptyFields map[string]omitEmptyTagMap
@@ -26,6 +27,7 @@ func makeFieldsParser(t reflect.Type, mapper FieldMapperFunc, useDefault bool) s
 	var once sync.Once
 	sf := &structFields{
 		fieldAlias:      map[string]string{},
+		fieldAs:         map[string]string{},
 		taggedFields:    map[string][]string{},
 		quotedFields:    map[string]struct{}{},
 		omitEmptyFields: map[string]omitEmptyTagMap{},
@@ -118,6 +120,13 @@ func (sf *structFields) parse(t reflect.Type, mapper FieldMapperFunc, prefix str
 				sf.quotedFields[alias] = struct{}{}
 			}
 		}
+
+		// Parse FieldAs.
+		fieldas := field.Tag.Get(FieldAs)
+
+		if fieldas != "" {
+			sf.fieldAs[alias] = fieldas
+		}
 	}
 
 	for _, field := range anonymous {