Hello Truong Pham,
I cannot be entirely sure where your problem lies without further explanation.
However, when you create a self referencing association within the entity model you have to generate the Database from the model in order to add the new properties to the database.
So if you aren't updating the database after modifying the model, perhaps that is causing the issue.
If not, please elaborate on your scenario and the steps you follow to implement it. Then we will be able to further assist you.
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