Let $(X,B,\mu)$ and $(Y,C,\nu)$ be probability spaces, and let $m$ be the product measure. Let $f:X \times Y \rightarrow [0,\infty )$ be a $B \otimes C $ measurable function, $1 < p < \infty$, and define $f_x(y):=f(x,y)$.

What is the most general condition on $f$ to make sure that: $||f||_{L_p(m)}=\int{||f_x||_{L_p(\nu)}}d\mu(x)$ ?

I know that if $\int{|f_x(y)|^p}d\nu(y)$ is constant there is equality. But is it the only case?