Support for readonly transient field #394

[wingsofovnia]

The issue #394 raises a very typical use case for fields that are not physically present in the table as a column, however, they are computed on reads.

For example, if I want to have a full_name property populated in type User struct { f_name string, s_name string, full_name string} for a table User = { f_name, s_name } the query

dbMap.SelectOne(&user, 
"select f_name, s_name, concat(f_name, s_name) full_name" from user...)

will successfully map the result as expected, however, the inserts will fail.

If I ignore full_name field via db:"-", inserts will work as expected, but reads will raise NoFieldInTypeError. Despite this is a "non-fatal error", it breaks the old codebase that expects no errors.

This change introduces a new field Tag "readonly" that behaves exactly as a transient field (db.go#readStructColumns maps such column to ColumnMap with Transient=true) , except gorp.go#columnToFieldIndex won't skip it for mapping select results.

Tested locally with postgres db.

@nelsam @hinet

[wingsofovnia]

CI fails on TestNullTime test that is not related to the change