You're seeing just the function primary_key-exclamation-mark, go back to Ecto.Adapters.DynamoDB.Info module for more information.
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primary_key!(repo, tablename)

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primary_key!(Ecto.Repo.t(), table_name_t()) ::
  {:primary, [String.t()]} | no_return()

Returns the primary key/ID for a table. It may be a single field that is a HASH, OR it may be the dynamoDB {HASH, SORT} type of index. we return {:primary, [index]} in a format described in normalise_dynamo_index!