This method is the basic mechanism to get a single row. It assumes
that the key specifies a unique row. If keyname is not specified
then the primary key for the table is used. If arg is a dictionary
then the value for the key is taken from it and it is modified to
include the new values, replacing existing values where necessary.
The OID is also put into the dictionary but in order to allow the
caller to work with multiple tables, the attribute name is munged
to make it unique. It consists of the string oid_ followed by
the name of the table.