grouped regresion in R

data = df explicitly uses the entire data frame df, ignoring any grouping. Use . to refer to the data that is piped in, which will let do use the groups. See the example at the bottom of ?do for reference:

## From ?do
by_cyl <- mtcars %>% group_by(cyl)
models <- by_cyl %>% do(mod = lm(mpg ~ disp, data = .))

Though, versions of dplyr > 1.0 will prefer using nest_by (also demonstrated on the ?do help page):

models <- mtcars %>%
  nest_by(cyl) %>%
  mutate(mod = list(lm(mpg ~ disp, data = data)))

models %>% summarise(broom::tidy(mod))