Symbolic Toolbox: How to substitute partial derivative resulting from chain rule?

Consider the following:

syms f(r,s) R(x,y) S(x,y) dfdr
dfdx = diff(f(R,S), x)

which gives as ouput:

dfdx =
D([1], f)(R(x, y), S(x, y))*diff(R(x, y), x) + D([2], f)(R(x, y), S(x, y))*diff(S(x, y), x)

Now, how can I subsitute

D([1], f)(R(x, y), S(x, y))

with

dfdr 

?

My attempt with

subs(dfdx, D([1], f)(R(x, y), S(x, y)), dfdr)

fails with the error message:

Error: ()-indexing must appear last in an index expression. 

Any help is appreciated, thanks!