feature request: unmatched="error" for the left table (x) in the left_join()

[idavydov]

When performing left_join() it would be nice to have an option to throw an error in case one of the rows in x didn't have a match.

Background

A common use-case is to have some kind of large metainformation table. You would like to join meta-information while making sure that every row of the original (left) table has exactly one match.

unmatched="error" in the left_join() will check if all the rows in y had a match.

Example

meta <- tribble(
  ~ Species, ~ Scientific.Name,
  "setosa", "Iris setosa",
  "virginica", "Iris virginica",
  # versicolor is missing
  "human", "Homo sapiens",
  "dog", "Canis lupus familiaris",
)
with_meta <- left_join(
  iris,
  meta,
  relationship = "many-to-one"
  # here we need something like unmatched = "x_error" , which will through an error because 
  # versicolor wasn't matched
)
# that's a work-around
stopifnot(any(is.na(with_meta$Scientific.Name)))

[DavisVaughan]

That's an inner join with unmatched = c("error", "drop") (i.e. error if an x row is unmatched but it is ok if a y row is unmatched).

library(dplyr, warn.conflicts = FALSE)

meta <- tribble(
  ~ Species, ~ Scientific.Name,
  "setosa", "Iris setosa",
  "virginica", "Iris virginica",
  # versicolor is missing
  "human", "Homo sapiens",
  "dog", "Canis lupus familiaris",
)

inner_join(
  iris,
  meta,
  relationship = "many-to-one",
  unmatched = c("error", "drop")
)
#> Joining with `by = join_by(Species)`
#> Error in `inner_join()`:
#> ! Each row of `x` must have a match in `y`.
#> ℹ Row 51 of `x` does not have a match.

It doesn't make sense to add this to left join because the "left" part of left join is a statement about what you do with the rows of x, i.e. the "left" part means: "keep all rows of x whether or not they have a match". In other words, when you use left_join() you always get an implicit unmatched = "keep" for x, and then you get to specify unmatched for y.

For inner join, you don't automatically get any unmatched behavior, so you get to specify unmatched for both x and y separately as needed