Rails-undefined method `id' for nil:NilClass

I am getting error undefined method `id' for nil:NilClass. I need to find id of oidValue.if id is nil for oidValue and i need to search oid2 table for get oidValue.How to check nil here.Below code is i am using.Not working if oidId is nil.

   oidId = Oid1.find_by(oid: oidvalue).id.to_s
    if oidId.nil?
      oidId = Oid2.find_by(oid: oidvalue).id.to_s
    end