Place the cumin in a small skillet and toast a minute or two over medium heat, until fragrant, then place in a medium bowl. Cut cucumber lengthwise in half. Use a small spoon and scrape out the seeds. Discard seeds. Coarsely grate the cucumber. Squeeze out as much moisture as you can from the cucumber, then add to the cumin seeds. Mix in the remaining ingredients.
Chill the raita at least 1 hour before serving.