choose()#

relationalai.InstanceProperty
InstanceProperty.choose(n: int, unique: bool = True) -> tuple[Producer]

Chooses n-combinations of values from a multi-valued property and returns them as a tuple (v1, v2, ..., vn) of Producer objects. By default, combinations are sorted in ascending order with no repeated values. If unique is False, all possible n-permutations with repeated values are produced. Must be used in a rule or query context.

Parameters#

NameTypeDescription
nintThe number of values to choose. Must be a positive integer.
uniqueboolIf True, the combination is sorted in ascending order with no repeated values. If False, all possible permutations (including repeated values) are produced. Default is True.

Returns#

A tuple of n Producer objects.

Example#

Use .choose() the get pairs, triples, or other combinations of values from a multi-valued property:

import relationalai as rai

# Create a model named "books" with Author and Book types.
model = rai.Model("books")
Author = model.Type("Author")
Book = model.Type("Book")

# Add some authors and books to the model.
with model.rule():
    Author.add(name="Kurt Vonnegut").books.add(Book.add(title="Cat's Cradle"))
    Author.add(name="Isaac Asimov").books.extend([
        Book.add(title="Foundation"),
        Book.add(title="Foundation and Empire")
    ])

# Get pairs of books written by the same author.
with model.query() as select:
    author = Author()
    book1, book2 = author.books.choose(2)
    response = select(book1.title, book2.title)

print(response.results)
# Output:
#         title                 title2
# 0  Foundation  Foundation and Empire

The model only contains one book for Kurt Vonnegut, so it is impossible to choose two unique books, which is why the dataframe returned by the query only contains titles of books by Isaac Asimov.

Set unique=False to get all possible permutations (including repeated values) of books by the same author:

with model.query() as select:
    author = Author()
    book1, book2 = author.books.choose(2, unique=False)
    response = select(book1.title, book2.title)

print(response.results)
# Output:
#                    title                 title2
# 0           Cat's Cradle           Cat's Cradle
# 1             Foundation             Foundation
# 2             Foundation  Foundation and Empire
# 3  Foundation and Empire             Foundation
# 4  Foundation and Empire  Foundation and Empire

.choose() should only be used with multi-valued properties. Calling it on a single-valued property, such a book’s title property, returns producers that produce no values:

with model.query() as select:
    title1, title2 = Book().title.choose(2)
    respose = select(title1, title2)

print(response.results)
# Output:
# Empty DataFrame
# Columns: []
# Index: []

Passing a 0 or a negative value to .choose() raises a ValueError:

with model.query() as select:
    title = Book().title.choose(0)
    response = select(title)

# Output:
# ValueError: Must choose a positive number of items

See Also#