# Placeholder¶

class getml.data.Placeholder(name: str, roles: Optional[Union[getml.data.roles_obj.Roles, Dict[str, List[str]]]] = None)[source]

Abstract representation of tables and their relations.

This class is an abstract representation of the DataFrame or View. However, it does not contain any actual data.

You might also want to refer to DataModel.

Args:
name (str):

The name used for this placeholder. This name will appear in the generated SQL code.

Examples:

This example will construct a data model in which the ‘population_table’ depends on the ‘peripheral_table’ via the ‘join_key’ column. In addition, only those rows in ‘peripheral_table’ for which ‘time_stamp’ is smaller or equal to the ‘time_stamp’ in ‘population_table’ are considered:

dm = getml.data.DataModel(
population_table.to_placeholder("POPULATION")
)

dm.POPULATION.join(
dm.PERIPHERAL,
on="join_key",
time_stamps="time_stamp"
)


If you want to add more than one peripheral table, you can use to_placeholder():

dm = getml.data.DataModel(
population_table.to_placeholder("POPULATION")
)

getml.data.to_placeholder(
PERIPHERAL1=peripheral_table_1,
PERIPHERAL2=peripheral_table_2,
)
)


If the relationship between two tables is many-to-one or one-to-one you should clearly say so:

dm.POPULATION.join(
dm.PERIPHERAL,
on="join_key",
time_stamps="time_stamp",
relationship=getml.data.relationship.many_to_one,
)


Please also refer to relationship.

If the join keys or time stamps are named differently in the two different tables, use a tuple:

dm.POPULATION.join(
dm.PERIPHERAL,
on=("join_key", "other_join_key"),
time_stamps=("time_stamp", "other_time_stamp"),
)


You can join over more than one join key:

dm.POPULATION.join(
dm.PERIPHERAL,
on=["join_key1", "join_key2", ("join_key3", "other_join_key3")],
time_stamps="time_stamp",
)


You can also limit the scope of your joins using memory. This can significantly speed up training time. For instance, if you only want to consider data from the last seven days, you could do something like this:

dm.POPULATION.join(
dm.PERIPHERAL,
on="join_key",
time_stamps="time_stamp",
memory=getml.data.time.days(7),
)


In some use cases, particularly those involving time series, it might be a good idea to use targets from the past. You can activate this using lagged_targets. But if you do that, you must also define a prediction horizon. For instance, if you want to predict data for the next hour, using data from the last seven days, you could do this:

dm.POPULATION.join(
dm.PERIPHERAL,
on="join_key",
time_stamps="time_stamp",
lagged_targets=True,
horizon=getml.data.time.hours(1),
memory=getml.data.time.days(7),
)


Please also refer to time.

If the join involves many matches, it might be a good idea to set the relationship to propositionalization. This forces the pipeline to always use a propositionalization algorithm for this join, which can significantly speed things up.

dm.POPULATION.join(
dm.PERIPHERAL,
on="join_key",
time_stamps="time_stamp",
relationship=getml.data.relationship.propositionalization,
)


Please also refer to relationship.

In some cases, it is necessary to have more than one placeholder on the same table. This is necessary to create more complicated data models. In this case, you can do something like this:

dm.add(
getml.data.to_placeholder(
PERIPHERAL=[peripheral_table]*2,
)
)

# We can now access our two placeholders like this:
placeholder1 = dm.PERIPHERAL[0]
placeholder2 = dm.PERIPHERAL[1]


If you want to check out a real-world example where this is necessary, refer to the CORA notebook.

Methods

 join(right[, on, time_stamps, relationship, …]) Joins another to placeholder to this placeholder. Expresses this placeholder and all of its descendants as a dictionary. Returns a list of this placeholder and all of its descendants.

Attributes