how to use group_by with a condition like if-then-else and apply dplyr philosophy

I need to group by variable x or variable y depending upon a condition. This is not happening when I use a magrittr pipe.

Consider a dataframe df1:

> df1


   seat_id student_id seat_state
1     1222        500          9
2      850        500          9
3      850        500          9
4     1225        500          9
5    16502        500          9
6    17792        500          9
7    17792        500          9
8     1219        501         10
9      847        501          9
10     847        501          9
11    1220        501          9
12   17785        501          9
13   17785        501          9
14    1214        502          9
15     842        502          9
16     842        502          9
17    1215        502          9
18    1211        503          9
19     839        503          9
20     839        503          9

Now suppose I want to summarise this in two ways 1. By student_id or 2. By seat_state Depending upon a variable

summary

The old and long way is

if (summary==1) df1 %>% group_by(student_id) %>% summarise(seats=n()) else if (summary==2) df1 %>% group_by(seat_state) %>% summarise(seats=n())

But there has to be a more compact way especially because I have several magrittr pipes coming after the summarise statement and therefore will double the size of the code.