I have created one project in spring boot which is book-store . I have one post service which is used to take details of book and author also and save that into db [both book and author at same time].

I want to create a logic where i can save new book for already existing author but i tried several ways and im unable to do.

I need a way to do this

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Here what i tried doing is i passed author email in path variable and passing a new book object as a json . i created a service which used to fetch particular author from db and i set that author object to my book object but it is saving that author object again and it is creating duplicate entry for that

author table:
here you can see duplicate entry of same author happend which i dont want
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book table:
book table is correct here new book got saved
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github link

this is my github link you can take a code and please help me to solve this issue . thanks in advance