The Stacks project

Proof. Say $I = (e_1, e_2, e_3, \ldots )$ with $e_ n$ an idempotent of $R$. After inductively replacing $e_{n + 1}$ by $e_ n + (1 - e_ n)e_{n + 1}$ we may assume that $(e_1) \subset (e_2) \subset (e_3) \subset \ldots $ and hence $I = \bigcup _{n \geq 1} (e_ n) = \mathop{\mathrm{colim}}\nolimits _ n e_ nR$. In this case

Note that the transition maps $e_{n + 1}M \to e_ nM$ are given by multiplication by $e_ n$ and are surjective. Hence by Algebra, Lemma 10.86.4 the functor $\mathop{\mathrm{Hom}}\nolimits _ R(I, M)$ is exact, i.e., $I$ is a projective $R$-module. $\square$