Question

Pretty straight forward problem. I simply want to return the affected row created by a SQL INSERT query so I can get data from it.

There’s a way I can do it already, I suppose, but I’m hoping the Mysql or Mysql2 gem provides some mechanism for me not having to make the second SELECT query.

The solution I’m leaning towards right now is something akin to:

"INSERT INTO table (col1) VALUES ('value');"

and then:

"SELECT cid FROM table ORDER BY cid DESC LIMIT 1;"

Since cid is the auto-increment index of the table (it’s InnoDB fyi), it will always be the largest cid value in the table until you do another INSERT.

Is there any mechanism in Mysql or Mysql2 to avoid having to make that second SELECT query?

Answer 1

MySQL2 has a last_id method. The documentation for that method is worthless but the implementation looks like this:

static VALUE rb_mysql_client_last_id(VALUE self) {
  GET_CLIENT(self);
  REQUIRE_OPEN_DB(wrapper);
  return ULL2NUM(mysql_insert_id(wrapper->client));
}

And the MySQL mysql_insert_id function does this:

Returns the value generated for an AUTO_INCREMENT column by the previous INSERT or UPDATE statement.

So you can do your INSERT and then get the last ID by calling the last_id method.

And BTW, your current approach:

SELECT cid FROM table ORDER BY cid DESC LIMIT 1;

is not safe if you’re in a multiprocess environment, consider this:

1. You INSERT a row.
2. Another process INSERTs a row.
3. You SELECT cid FROM table ORDER BY cid DESC LIMIT 1 and get the ID from (2).

You could SELECT last_insert_id() though, last_insert_id() is session-specific so you don’t have to worry about other processes when using it.