Small rails app to demonstrate belongs_to preload bug.
Open rails console and type
Lead.create name: "ranu", email: "abc@xyz.com", agent_code: "susa"
Agent.create name: "Susa", email: "susa@test.com", code: "SUSA"
a = Lead.includes(:agent).where(id: 1).first
This gives NoMethodError (undefined method first' for nil:NilClass)