Let $R$ be a commutative ring with identity and $M$ be an $R$-module. I have trouble understanding the restriction map in the definition of the sheaf of $\mathcal{O}_X$ modules. Explicitly, let $f,g\in R$ s.t. $D(g)\subseteq D(f)$, then what is the map $M_f\to M_g$. My main difficulty is understanding what kind of module homomorphism to expect, since the left hand side module is over $R_f$ while the one on the right is over $R_g$. Most books I flipped through, said "it is defined similarly" referring to the definition of the restriction maps in the structure sheaf of Spec$R$
restriction map in a Sheaf of $\mathcal{O}_X$ modules
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algebraic-geometry
commutative-algebra
category-theory