Making an aldehyde from a carbanion by attacking a carbonyl

This is a problem. Attacking a carbonyl which doesn't have a good leaving group will create an alcohol:

While attacking a carbonyl with a good leaving group will result in it being attacked twice:

What we need is something which forms a stable tetrahedral intermediate, yet which has a group which leaves during the acid workup. By leaving at the same time as the acid workup, the organometallic is destroyed at the same time, preventing another attack.

A solution is DMF, or dimethylformamide:

In otherwords, adding DMF to an organometallic will replace the metal with an aldehyde group. Another example:

You might wonder what makes the tetrahedral intermediate stable, since amine anions can occasionally act as leaving groups. I don't know the answer.