  Let $(g,X)$ be a Sasaki-Ricci soliton on a Sasakian manifold $S$. We prove
that if $(S,g)$ admits a local Sasakian immersion in a Sasakian space form
$S(N,c)$ of constant $\phi$-sectional curvature $c$, then $S$ is
$\eta$-Einstein and its $\eta$-Einstein constants are rational. Moreover, if
$c\leq -3$, $S$ is locally equivalent to the Sasakian space form $S(n,c)$ and
its $\eta$-Einstein constants are determined by $c$. Further results are
obtained in the compact setting, i.e. when $c>-3$, under additional hypotheses.
