Mongo.Ecto.Helpers.change_array

You're seeing just the function change_array, go back to Mongo.Ecto.Helpers module for more information.
Link to this function

change_array(idx, field \\ "", value)

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Specs

change_array(pos_integer(), String.t(), term()) :: Mongo.Ecto.ChangeArray.t()

Allows updating only a fragment of a nested document inside an array

Usage in queries

MyRepo.update_all(Post,
  set: [comments: change_array(0, "author", "NewName")])