[Rails] error in runtime controller#view

m getting an error "Called id for nil, which would mistakenly be 4 -- if you really wanted the id of nil, use object_id"

when i'm creating new record,like a topic has many discussions,when i'm creating a new discussions m getting that error..i already assign the topic id as foreign key in discussions controller....please sort out this problem.if u getting the same..how you rectify those errors..and in how many cases this errors are prone to occur???

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