FAQ
===

Whenever you find out something that you were surprised by or needed some non-trivial
thinking, please add it here.

How do I define additional unique keys in a ``dy.Schema``?
----------------------------------------------------------

By default, ``dataframely`` only supports defining a single non-nullable (composite) primary key in ``dy.Schema``.
However, in some scenarios it may be useful to define additional unique keys (which support nullable fields and/or which are additionally unique).

Consider the following example, which demonstrates two rules: one for validating that a field is entirely unique, and another for validating that a field, when provided, is unique.

::

    class UserSchema(dy.Schema):
        user_id = dy.UInt64(primary_key=True, nullable=False)
        username = dy.String(nullable=False)
        email = dy.String(nullable=True)  # Must be unique, or null.

        @dy.rule(group_by=["username"])
        def unique_username() -> pl.Expr:
            """Username, a non-nullable field, must be total unique."""
            return pl.len() == 1

        @dy.rule()
        def unique_email_or_null() -> pl.Expr:
            """Email must be unique, if provided."""
            return pl.col("email").is_null() | pl.col("email").is_unique()