R/mock-db.R

In ropensci/dittodb: A Test Environment for Database Requests

#' Run DBI queries against a mocked database
#'
#' Wrap a chunk of code in `with_mock_db()` to use mocked databases that will
#' use fixtures instead of connecting to a real database. Alternatively, you can
#' start and stop using a mocked database with `start_mock_db()` and
#' `stop_mock_db()` respectively.to execute the whole thing without needing to
#' remember to stop the mocking. When testing with `dittodb`, it will look for
#' fixtures in all entries of [`db_mock_paths`].
#'
#' You only need to use one approach: either use `start_mock_db()` to start
#' using mocks and then `stop_mock_db()` to stop or use `with_mock_db()` wrapped
#' around the code you want to execute against the mocked database. You don't
#' need to (and should not) use both at the same time. Generally
#' `with_mock_db()` is preferred because it is slightly safer and you don't have
#' to remember to `stop_mock_db()` when you're done. However, it is easier to
#' step through tests interactively using `start_mock_db()`/`stop_mock_db()`.
#'
#' Connections should be made after `start_mock_db()` if you're using that
#' function or they should be made inside of `with_mock_db()` if you're using
#' that function because `dittodb` uses the database name (given in `dbname` or
#' `Database` argument of [`dbConnect`] depending on the driver) to separate
#' different fixtures. For ODBC connections with only a dsn provided, the dsn is
#' used for this directory.
#'
#' @param expr the expression to execute
#'
#' @return nothing
#' @export
#'
#' @name mockdb
#'
#' @examples
#' # Add the mocks included with dittodb to the db_mock_paths to use them below
#' db_mock_paths(system.file("nycflight_mocks", package = "dittodb"), last = TRUE)
#'
#' if (check_for_pkg("RSQLite", message) & check_for_pkg("testthat", message)) {
#'   # using  `with_mock_db()`
#'   with_mock_db({
#'     con <- dbConnect(
#'       RSQLite::SQLite(),
#'       dbname = "nycflights"
#'     )
#'
#'     testthat::test_that("We get one airline", {
#'       one_airline <- dbGetQuery(
#'         con,
#'         "SELECT carrier, name FROM airlines LIMIT 1"
#'       )
#'       testthat::expect_s3_class(one_airline, "data.frame")
#'       testthat::expect_equal(nrow(one_airline), 1)
#'       testthat::expect_equal(one_airline$carrier, "9E")
#'       testthat::expect_equal(one_airline$name, "Endeavor Air Inc.")
#'     })
#'
#'     dbDisconnect(con)
#'   })
#'
#'   # using `start_mock_db()` and `stop_mock_db()`
#'   start_mock_db()
#'   con <- dbConnect(
#'     RSQLite::SQLite(),
#'     dbname = "nycflights"
#'   )
#'
#'   testthat::test_that("We get one airline", {
#'     one_airline <- dbGetQuery(
#'       con,
#'       "SELECT carrier, name FROM airlines LIMIT 1"
#'     )
#'     testthat::expect_s3_class(one_airline, "data.frame")
#'     testthat::expect_equal(nrow(one_airline), 1)
#'     testthat::expect_equal(one_airline$carrier, "9E")
#'     testthat::expect_equal(one_airline$name, "Endeavor Air Inc.")
#'   })
#'
#'   dbDisconnect(con)
#'   stop_mock_db()
#' }
with_mock_db <- function(expr) {
  start_mock_db()
  on.exit(stop_mock_db())

  return(eval(expr, parent.frame()))
}

#' @rdname mockdb
#' @export
start_mock_db <- function() {
  trace_dbi(
    "dbConnect",
    # This changes the result of standardGeneric to be our mocked connection,
    # rather than real S4 dispatch
    tracer = quote({
      mock_connection <- dbMockConnect(drv, ...)
      standardGeneric <- function(...) {
        return(mock_connection)
      }
    }),
    where_list = list(sys.frame(), asNamespace("DBI"))
  )
}

#' @rdname mockdb
#' @export
stop_mock_db <- function() {
  mocked_already <- any(
    inherits(
      try(get("dbConnect", sys.frame()), silent = TRUE),
      c("functionWithTrace", "standardGenericWithTrace")
    ), inherits(
      try(get("dbConnect", asNamespace("DBI")), silent = TRUE),
      c("functionWithTrace", "standardGenericWithTrace")
    )
  )

  if (!mocked_already) {
    message("There is no mock database being used.")
    return(invisible(NULL))
  }

  safe_untrace("dbConnect", sys.frame())
  safe_untrace("dbConnect", asNamespace("DBI"))

  return(invisible(NULL))
}