def apply_join(query: Select, table: Table, join_table: Table, join: TableJoin):
"""
Performs a inner or outer join between two tables on a given query object.
TODO: enable multiple joins
:param query: A SQLAlchemy select object.
:param table: The Table we are joining from.
:param join_table: The Table we are joining to.
:param join: The Join object describing how to join the tables.
:return: A SQLAlchemy select object modified to join two tables.
"""
error_msg = 'Invalid join, "{}" is not a column on table "{}"'
join_conditions = []
for column_pair in join.column_pairs:
from_col = table.columns.get(column_pair.from_column)
to_col = join_table.columns.get(column_pair.to_column)
if from_col is None:
raise ValueError(error_msg.format(column_pair.from_column, table.name))
if to_col is None:
raise ValueError(error_msg.format(column_pair.to_column, join_table.name))
join_conditions.append(from_col == to_col)
return query.select_from(table.join(join_table, onclause=and_(*join_conditions), isouter=join.outer_join))
