entc/select: add selection api

Summary: Pull Request resolved: https://github.com/facebookincubator/ent/pull/27

Reviewed By: alexsn

Differential Revision: D17164536

fbshipit-source-id: f5caa80b05aff1ef8ec30d7a53256b00380d018c

entc/gen/template/dialect/sql/select.tmpl

@@ -0,0 +1,26 @@
+{{/*
+Copyright 2019-present Facebook Inc. All rights reserved.
+This source code is licensed under the Apache 2.0 license found
+in the LICENSE file in the root directory of this source tree.
+*/}}
+
+{{ define "dialect/sql/select" }}
+{{ $builder := pascal $.Scope.Builder }}
+{{ $receiver := receiver $builder }}
+
+func ({{ $receiver }} *{{ $builder }}) sqlScan(ctx context.Context, v interface{}) error {
+	rows := &sql.Rows{}
+	query, args := {{ $receiver }}.sqlQuery().Query()
+	if err := {{ $receiver }}.driver.Query(ctx, query, args, rows); err != nil {
+		return err
+	}
+	defer rows.Close()
+	return sql.ScanSlice(rows, v)
+}
+
+
+func ({{ $receiver }} *{{ $builder }}) sqlQuery() sql.Querier {
+	view := "{{ $.Package }}_view"
+	return sql.Select({{ $receiver }}.fields...).From({{ $receiver }}.sql.As(view))
+}
+{{ end }}