Select 5 of each distinct value

Select 5 of each distinct value

I have the following table in PostgreSQL:
| a | b | c |
===================
| 'w' | 2 | 3 |
| 'w' | 7 | 2 |
| 'w' | 8 | 1 |
| 'w' | 3 | 6 |
| 'w' | 0 | 8 |
| 'w' | 2 | 9 |
| 'w' | 2 | 9 |
| 'z' | 4 | 9 |
| 'z' | 0 | 9 |
| 'z' | 0 | 8 |
| 'z' | 3 | 6 |
| 'z' | 2 | 7 |
| 'z' | 3 | 1 |
| 'z' | 3 | 2 |
| 'z' | 3 | 3 |
I want to select all records, but limit them to 5 records for each
distinct value in column a.
So the result would look like:
| a | b | c |
===================
| 'w' | 2 | 3 |
| 'w' | 7 | 2 |
| 'w' | 8 | 1 |
| 'w' | 3 | 6 |
| 'w' | 0 | 8 |
| 'z' | 4 | 9 |
| 'z' | 0 | 9 |
| 'z' | 0 | 8 |
| 'z' | 3 | 6 |
| 'z' | 2 | 7 |
What is the most effecient way to achieve that in RoR? Thanks!