# Class dm and basic operations

#### 2025-07-02

The goal of the {dm} package and the `dm` class that comes with it, is to make your life easier when you are dealing with data from several different tables.

Let’s take a look at the `dm` class.

## Class `dm`

The `dm` class consists of a collection of tables and metadata about the tables, such as

- the names of the tables
- the names of the columns of the tables
- the primary and foreign keys of the tables to link the tables together
- the data (either as data frames or as references to database tables)

All tables in a `dm` must be obtained from the same data source; csv files and spreadsheets would need to be imported to data frames in R.

## Examples of `dm` objects

There are currently three options available for creating a `dm` object. The relevant functions for creating `dm` objects are:

1. `dm()`
2. `as_dm()`
3. `new_dm()`
4. `dm_from_con()`

To illustrate these options, we will now create the same `dm` in several different ways. We can use the tables from the well-known {nycflights13} package.

### Pass the tables directly

Create a `dm` object directly by providing data frames to `dm()`:

    library(nycflights13)
    library(dm)
    dm(airlines, airports, flights, planes, weather)

### Start with an empty `dm`

Start with an empty `dm` object that has been created with `dm()` or `new_dm()`, and add tables to that object:

    library(nycflights13)
    library(dm)
    empty_dm <- dm()
    empty_dm
    dm(empty_dm, airlines, airports, flights, planes, weather)

### Coerce a list of tables

Turn a named list of tables into a `dm` with `as_dm()`:

    as_dm(list(
      airlines = airlines,
      airports = airports,
      flights = flights,
      planes = planes,
      weather = weather
    ))

### Turn tables from a `src` into a `dm`

Squeeze all (or a subset of) tables belonging to a `src` object into a `dm` using `dm_from_con()`:

    sqlite_con <- dbplyr::nycflights13_sqlite()

    flights_dm <- dm_from_con(sqlite_con)
    flights_dm

The function `dm_from_con(con, table_names = NULL)` includes all available tables on a source in the `dm` object. This means that you can use this, for example, on a postgres database that you access via `DBI::dbConnect(RPostgres::Postgres())` (with the appropriate arguments `dbname`, `host`, `port`, …), to produce a `dm` object with all the tables on the database.

### Low-level construction

Another way of creating a `dm` object is calling `new_dm()` on a list of `tbl` objects:

    base_dm <- new_dm(list(
      airlines = airlines,
      airports = airports,
      flights = flights,
      planes = planes,
      weather = weather
    ))
    base_dm

This constructor is optimized for speed and does not perform integrity checks. Use with caution, validate using `dm_validate()` if necessary.

    dm_validate(base_dm)

## Access tables

We can get the list of tables with `dm_get_tables()` and the `src` object with `dm_get_con()`.

In order to pull a specific table from a `dm`, use:

    flights_dm[["airports"]]

But how can we use {dm}-functions to manage the primary keys of the tables in a `dm` object?

## Primary keys of `dm` objects

Some useful functions for managing primary key settings are:

1. `dm_add_pk()`
2. `dm_get_all_pks()`
3. `dm_rm_pk()`
4. `dm_enum_pk_candidates()`

If you created a `dm` object according to the examples in “Examples of `dm` objects”, your object does not yet have any primary keys set. So let’s add one.

We use the `nycflights13` tables, i.e. `flights_dm` from above.

    dm_has_pk(flights_dm, airports)
    flights_dm_with_key <- dm_add_pk(flights_dm, airports, faa)
    flights_dm_with_key

The `dm` now has a primary key:

    dm_has_pk(flights_dm_with_key, airports)

To get an overview over all tables with primary keys, use `dm_get_all_pks()`:

    dm_get_all_pks(flights_dm_with_key)

Remove a primary key:

    dm_rm_pk(flights_dm_with_key, airports) %>%
      dm_has_pk(airports)

If you still need to get to know your data better, and it is already available in the form of a `dm` object, you can use the `dm_enum_pk_candidates()` function in order to get information about which columns of the table are unique keys:

    dm_enum_pk_candidates(flights_dm_with_key, airports)

The `flights` table does not have any one-column primary key candidates:

    dm_enum_pk_candidates(flights_dm_with_key, flights) %>% dplyr::count(candidate)

`dm_add_pk()` has a `check` argument. If set to `TRUE`, the function checks if the column of the table given by the user is unique. For performance reasons, the default is `check = FALSE`. See also [dm_examine_constraints()] for checking all constraints in a `dm`.

    try(
      dm_add_pk(flights_dm, airports, tzone, check = TRUE)
    )

## Foreign keys

Useful functions for managing foreign key relations include:

1. `dm_add_fk()`
2. `dm_get_all_fks()`
3. `dm_rm_fk()`
4. `dm_enum_fk_candidates()`

Now it gets (even more) interesting: we want to define relations between different tables. With the `dm_add_fk()` function you can define which column of which table points to another table’s column.

This is done by choosing a foreign key from one table that will point to a primary key of another table. The primary key of the referred table must be set with `dm_add_pk()`. `dm_add_fk()` will find the primary key column of the referenced table by itself and make the indicated column of the child table point to it.

    flights_dm_with_key %>% dm_add_fk(flights, origin, airports)

This will throw an error:

    try(
      flights_dm %>% dm_add_fk(flights, origin, airports)
    )

Let’s create a `dm` object with a foreign key relation to work with later on:

    flights_dm_with_fk <- dm_add_fk(flights_dm_with_key, flights, origin, airports)

What if we tried to add another foreign key relation from `flights` to `airports` to the object? Column `dest` might work, since it also contains airport codes:

    try(
      flights_dm_with_fk %>% dm_add_fk(flights, dest, airports, check = TRUE)
    )

Checks are opt-in and executed only if `check = TRUE`. You can still add a foreign key with the default `check = FALSE`. See also `dm_examine_constraints()` for checking all constraints in a `dm`.

Get an overview of all foreign key relations with`dm_get_all_fks()`:

    dm_get_all_fks(dm_nycflights13(cycle = TRUE))

Remove foreign key relations with `dm_rm_fk()` (parameter `columns = NULL` means that all relations will be removed, with a message):

    try(
      flights_dm_with_fk %>%
        dm_rm_fk(table = flights, column = dest, ref_table = airports) %>%
        dm_get_all_fks(c(flights, airports))
    )

    flights_dm_with_fk %>%
      dm_rm_fk(flights, origin, airports) %>%
      dm_get_all_fks(c(flights, airports))

    flights_dm_with_fk %>%
      dm_rm_fk(flights, columns = NULL, airports) %>%
      dm_get_all_fks(c(flights, airports))

Since the primary keys are defined in the `dm` object, you do not usually need to provide the referenced column name of `ref_table`.

Another function for getting to know your data better (cf. `dm_enum_pk_candidates()` in “Primary keys of `dm` objects”) is `dm_enum_fk_candidates()`. Use it to get an overview over foreign key candidates that point from one table to another:

    dm_enum_fk_candidates(flights_dm_with_key, weather, airports)