PG Array in declarative mapping

[Samuel Killin]

I'm struggling to define a SQLAlchemy column for a postgres string array using the new declarative mapping pattern in 2.0.

I would expect something like this to work:
my_col: Mapped[list[str]] = mapped_column()

However, this throws with:
sqlalchemy.exc.ArgumentError: Could not locate SQLAlchemy Core type for Python type list[str] inside the 'my_col' attribute Mapped annotation

Any ideas?

[Mike Bayer]

hey there -

a list[str] can be lots of different kinds of datatype, like JSON, custom array types, and PG ARRAY, which is specific to PostgreSQL.  so for non-obvious datatypes like this you indicate them explicitly:

from sqlalchemy.dialects.postgresql import ARRAY

from sqlalchemy import String

col: Mapped[list[str]] = mapped_column(ARRAY(String))

hoping this is clear from the doc at https://docs.sqlalchemy.org/en/20/orm/declarative_tables.html#using-annotated-declarative-table-type-annotated-forms-for-mapped-column which illustrates several of the mapped_column() with explicit datatype and also states "If the mapped_column() construct indicates an explicit type as passed to the mapped_column.__type argument, then the given Python type is disregarded".

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