Ruby on Rails

Please help me to understand why this piece of rspec code is behaving this way. I am setting product_id to my record, and it is asserting true, but yet record.product not returning anything.

product = FactoryGirl.create(:product)

post :create, {:tshirt => FactoryGirl.attributes_for(:tshirt).stringify_keys.merge("product_id"=> product.id.to_s) } #successful creation
expect(Tshirt.last.product_id).to eq(product.id)  # asserts true

puts Tshirt.last.product #prints nothing

expect(Tshirt.last.product.size).to eq(product.size)  # exception!!! it says, product = nil

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 Md. Sadaf Noor (@sadaf2605)
 www.sadafnoor.com