Ecto.Adapter.Schema.autogenerate

You're seeing just the callback autogenerate, go back to Ecto.Adapter.Schema module for more information.
Link to this callback

autogenerate(field_type)

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Specs

autogenerate(field_type :: :id | :binary_id | :embed_id) :: term() | nil

Called to autogenerate a value for id/embed_id/binary_id.

Returns the autogenerated value, or nil if it must be autogenerated inside the storage or raise if not supported.