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The error "populate is not a function" in Mongoose indicates that you are calling the populate method on an object that is not a valid Mongoose document or query. The populate method is used to retrieve related documents from other collections in MongoDB, and it can only be invoked on valid query objects or documents.
Possible causes of this error include:
- You are calling the populate method on a plain JavaScript object that is not a Mongoose query or document.
- You are calling the populate method on a Mongoose query that has not been executed yet (i.e., you have not called exec() on it).
- You are calling the populate method on a Mongoose document that does not have a reference field that can be populated.
To fix this error, make sure you are calling the populate method on a valid Mongoose query or document that has a reference field that can be populated.