# Getting Started with {dittodb}

#### 2022-06-17

{dittodb} is designed to make it easy and fun to test functions that interact with a database. It works by looking for mock responses for each query you send while you run your tests and will seamlessly pretend that those mocks were provided by the database connection without needing a connection at all.

## Our function mean_delays()

To get started, imagine that we are working on a package that queries a database that consists of the nycflights13 data. We have the following function which takes a column to aggregate by and returns a dataframe with that column and the mean delay for groups based on the values in the column name given.

library(DBI)

mean_delays <- function(group_col) {
con <- dbConnect(
dbname = "nycflights"
)
on.exit(dbDisconnect(con))

query <- glue::glue(
"SELECT {group_col}, AVG(arr_delay) as mean_delay from nycflights13.flights ",
"WHERE arr_delay > 0 GROUP BY {group_col}"
)

return(dbGetQuery(con, query))
}

### RPostgres

library(DBI)

mean_delays <- function(group_col) {
con <- dbConnect(
RPostgres::Postgres(),
dbname = "nycflights"
)
on.exit(dbDisconnect(con))

query <- glue::glue(
"SELECT {group_col}, AVG(arr_delay) as mean_delay from nycflights13.flights ",
"WHERE arr_delay > 0 GROUP BY {group_col}"
)

return(dbGetQuery(con, query))
}

### RSQLite

library(DBI)

mean_delays <- function(group_col) {
con <- dbConnect(RSQLite::SQLite())
on.exit(dbDisconnect(con))

query <- glue::glue(
"SELECT {group_col}, AVG(arr_delay) as mean_delay from nycflights13.flights ",
"WHERE arr_delay > 0 GROUP BY {group_col}"
)

return(dbGetQuery(con, query))
}

If we give it the column "month", we get the following dataframe:

mean_delays("month")
#>    month mean_delay
#> 1      1   34.47749
#> 2      2   33.68921
#> 3      3   40.57166
#> 4      4   42.73958
#> 5      5   41.88586
#> 6      6   53.73827
#> 7      7   53.95152
#> 8      8   39.51294
#> 9      9   38.80555
#> 10    10   29.03665
#> 11    11   27.48459
#> 12    12   39.72725

Great, now that we have our function we want to test it to make sure it is operating as expected. Normally, we could write something like:

library(testthat)

test_that("mean_delays()", {
out <- mean_delays("month")
expect_named(out, c("month", "mean_delay"))
expect_equal(dim(out), c(12, 2))
})

And this works just fine if we only ever run your tests locally, but if we want to run our tests with a Continuous Integration system (and yes, we want to do that!), this won’t work without first setting up our production database of flights. For our tests, we don’t actually need to connect to the database and get new data (and, in fact, that would make some tests fail erroneously suddenly if the underlying changed). Instead, what we want is to take a snapshot of what happens when running the test code, and then be able to use that snapshot when we run tests later. These snapshots are frequently called fixtures (though you might hear people use other names like stubs or mocks).

# Recording fixtures

We can record fixtures of the database interactions with the commands start_db_capturing(), run the functions we want to record, and then stop recording with stop_db_capturing().

start_db_capturing()
out <- mean_delays("month")
stop_db_capturing()

This will write a new folder (by default in ./tests/testthat/) with the name of the database (here: nycflights) and then write one file with the name SELECT-e53189.R which is the fixture for this example. This SELECT-* file contains the data that was received from the database for use in tests.

# with_mock_db()

Now that we have a fixture, we can use that fixture by wrapping our call that includes a database interaction with the function with_mock_db(). This will look for fixtures and use those.

with_mock_db(
mean_delays("month")
)
#>    month mean_delay
#> 1      1   34.47749
#> 2      2   33.68921
#> 3      3   40.57166
#> 4      4   42.73958
#> 5      5   41.88586
#> 6      6   53.73827
#> 7      7   53.95152
#> 8      8   39.51294
#> 9      9   38.80555
#> 10    10   29.03665
#> 11    11   27.48459
#> 12    12   39.72725

So, now we can write our tests like:

library(testthat)
library(dittodb)

with_mock_db(
test_that("mean_delays()", {
out <- mean_delays("month")
expect_named(out, c("month", "mean_delay"))
expect_equal(dim(out), c(12, 2))
})
)
#> Test passed 🥳
#> [1] TRUE

When placed inside of with_mock_db(...) a call to mean_delays("month") will return what we saved as our fixture as if it had actually connected to the database without needing the database to be installed, reachable, operational, or to exist at all anywhere.

If we wanted to test that a day-based aggregation works, we can! Although we will have to make a new fixture. First we would run the following interactively:

start_db_capturing()
out <- mean_delays("day")
stop_db_capturing()

This will create a new file (SELECT-16d120.R) which contains the response when aggregating by day. dittodb saves each database interaction with a hash of the query that is sent, so that a number of different responses from a database can be saved and the correct one will be used when called inside of with_mock_db(...). So now, we could write our new test with:

with_mock_db(
test_that("mean_delays()", {
out <- mean_delays("day")
expect_named(out, c("day", "mean_delay"))
expect_equal(dim(out), c(31, 2))
})
)
#> Test passed 😸
#> [1] TRUE

# Getting setup to use {dittodb}

Use the function dittodb::use_dittodb() to easily get started using {dittodb}. It will add {dittodb} to Suggests in the DESCRIPTION file and add library(dittodb) to tests/testthat/helper.R.

# Things to be careful about

There are a few things to be careful about when using dittodb.

## When to call dbConnect()

Always call dbConnect() inside of with_mock_db(...). You can make as many calls as you want to the mock database inside of a with_mock_db(...), but you should always make sure that you connect to the database inside of and not outside of with_mock_db(...). This is because when you “connect” to the mock database, a few variables are set that tell dittodb where to look for mocks. It’s less important (though still a good idea) to call dbDisconnect() inside of with_mock_db(). This is also true when recording fixtures with start_db_recording(), you should start the recording and then call dbConnect().

## Query size

Recording fixtures saves the whole query to disk in a relatively inefficient way (from a data storage perspective), so be careful with what you save. And you’ll want to not save extremely large results if at all possible. This is also a best-practice for writing tests anyway: you should have mocks that are as minimal as possible to test the functionality you need to. Minimal mocks make it easier to change things that aren’t relevant to the test (you don’t have to change the way data is represented if it’s not important to what you’re testing) and it makes your tests run faster.

There are a number of advanced features that might be useful. However they take a bit of configuration to use.

## Specify a new path

You can control where mocks are read from (when you’re using with_mock_db(...)) as well as where they are written to (when using start_db_capturing()). To do this, use the function db_mock_paths().

You can see what paths are being used by calling db_mock_paths() with no arguments. dittodb will look for mocks in each path starting with the first one. When recording mocks, dittodb always uses the first path that is returned by db_mock_paths().

You can add a new path by calling db_mock_paths("new/path/here") which will add the path provided to the top of the list of paths to use.

## Redacting

Sometimes (much? most? of the time!) there is sensitive data in your database that you don’t actually want to put into your test fixtures. {dittodb} allows you to specify columns that should always be redacted by specifying them like so:

start_db_capturing(redact_columns = c("sensitive_column", "other_sensitive_column"))

This will always redact the columns “sensitive_column” and “other_sensitive_column” every time a query is recorded that includes either. The redactor replaces every value in the column with a standard value (for example “[redacted]” for characters, 9 for numerics, 1988-10-11T17:00:00 for date times) see redact_columns() for more information.

## You, too, can write a fixture!

When we use start_db_recording() to record fixtures, we are creating what some people call fixtures (though other terms for these abound). These are files that are used during testing to represent and provide some data or state necessary to execute the test. In the case of dittodb, these files contain the data that dittodb uses when it pretends to be a live database. During recording, each query that is sent to the database gets a unique identifier (the first 6 digits of the hash of the query) and when the response is received, that response is saved to a file with the first SQL (Structured Query Language) verb (e.g. SELECT), a dash, and the hash using the dput() function. This lets you craft a fixture that tests exactly what you need to without having extraneous rows or columns that might not be relevant.

You can save our own responses for queries by getting figuring out the hash (the easiest way to do this now is to write the test that you want to create a fixture for, run it and see the error message that looks something like “Couldn’t find the file nycflights/SELECT-16d120.R in any of the mock directories.” and use the filename from there.) and then saving the dataframe that you want the test to use with the command dput(df, file = "nycflights/SELECT-16d120.R", control = c("all", "hexNumeric")) (if the dataframe you want to save is df and we are using the path we saw in the example error message). And you’ve created your own fixture!

You can also take the approach of recording fixtures and then editing them manually to pare them down. The workflow for that would be something like:

# read in the recorded fixture
df_fixt <- source("nycflights/SELECT-16d120.R", keep.source = FALSE)\$value

# filter out anything after february and all days after the 9th of the month
df_fixt <- dplyr::filter(df_fixt, month <= 2 & day < 10)

# save the fixture for use in tests
dput(df_fixt, file = "nycflights/SELECT-16d120.R", control = c("all", "hexNumeric"))