sample_n_groups.R

sample_n_groups <- function(df, n, group_keys = as.character(groups(df))) {
  # Remember original grouping
  group_keys_orig <- as.character(groups(df))
  stopifnot(length(group_keys) != 0)

  df_sub <- df %>% 
    # Keep just the grouping columns
    group_by_(.dots = group_keys) %>% 
    select(one_of(group_keys)) %>% 
    distinct %>% 
    ungroup %>% 
    # Sample the grouping keys
    sample_n(n, replace = FALSE)

  # Keep just the sampled groups
  df %>% 
    semi_join(df_sub, by = group_keys) %>% 
    # Restore original grouping
    group_by_(.dots = group_keys_orig)
}

not robust against columns named with names wrapped in quotes like df[, '"Subj"']