Handling the case where there are no query results

[shankari]

If there are no query results, then many functions break. For the most recent change (#17), these were:

normalise_uuid
summarise_server_calls
tidy_server_calls

A workaround is to add the following short-circuit code to the beginning of each function.

+  if (nrow(.data) == 0) {
+    return(.data);
+  }

The query results can return no values early in the data collection; when there have been no trips yet, for example, or the user has not synced with the server yet.

I am not an R expert, so was curious about how you plan to handle the case in which there are no query results. Instead of adding short-circuit code to each function, is there a way to create a blank data table at the query level?

[asiripanich]

My proposal is that if there are no participants, the dashboard should just return an error for now. But if there are no trips, the easiest way, that I can think of right now, to solve this without running into a fatal error is to return a emptied data.table that has all the columns needed by the other functions. This is equivalent to subsetting a data.table with data[0, ].

[shankari]

@asiripanich there will be participants because we register during onboarding. But there might not be trips or server calls corresponding to /usercache/get and /usercache/put.

[shankari]

the commit looks good to me

[asiripanich]

This still needs to be patched.

[shankari]

I am now getting a new error with the dashboards for the new deployments

Running: mod_load_data_server
About to load trips
Finished query, about to clean trips
Warning: Error in [[<-.data.frame: replacement has 0 rows, data has 13
  [No stack trace available]
Warning in min(data_r$participants$update_ts) :
  no non-missing arguments to min; returning Inf
Warning in min(data_r$participants$update_ts) :
  no non-missing arguments to min; returning Inf
Warning: Error in UseMethod: no applicable method for 'transmute' applied to an object of class "NULL"
  [No stack trace available]
Warning: Error in UseMethod: no applicable method for 'transmute' applied to an object of class "NULL"
  [No stack trace available]
Warning in grSoftVersion() :
  unable to load shared object '/usr/local/lib/R/modules//R_X11.so':
  libXt.so.6: cannot open shared object file: No such file or directory
Warning: Error in FUN: object 'n_days' not found
  [No stack trace available]
Warning: Error in FUN: object 'curr_platform' not found
  [No stack trace available]
Warning: Error in drop_list_columns: is.data.frame(.data) is not TRUE
  80: <Anonymous>
Anonymize trajectories
Warning: Error in anonymize_uuid: is.data.frame(.data) is not TRUE
  69: <Anonymous>
Warning: Error in anonymize_uuid: is.data.frame(.data) is not TRUE
  81: <Anonymous>
disconnecting from the emission collections..

[shankari]

the key seems to be this error in tidy_cleaned_trips

Error in [[<-.data.frame: replacement has 0 rows, data has 13

there are two users with 11 and 2 trips. So we get 13 trips correctly. Not quite sure why they are getting filtered out.

[shankari]

so I loaded a bunch of other trips from two test users and the dashboard works fine now. I think that the problem might occur if there are no trips with labels. I have attached a mongodump that can be used to reproduce the problem and test the fix.
mongodump_13_trips_no_work.tar.gz

[asiripanich]

the key seems to be this error in tidy_cleaned_trips

Error in [[<-.data.frame: replacement has 0 rows, data has 13

there are two users with 11 and 2 trips. So we get 13 trips correctly. Not quite sure why they are getting filtered out.

@allenmichael099 is fixing this in #46.