Let M be an R-module,where R is a hereditary(or cohomological dimension less or equal to 1).Take E(R) to be injective hull of R, then we have the essential extension i:R^I--->E(R)^I (product I times)

and we also have p:R^I--->>M is epimorpshim. Then I take the push forward of these two morphisms i and p, denote the push out by (N,f,g),where f:M---->N and g:E(R)^I--->>N)

it is clear that N is an injective module(because it is image of E(R)^I and R is hereditary) and M--->N is injective morphism. However, N is not necessarily injective hull of M because in general, essential extension does not commutes with colimit.

My question is can we give some conditions to R or other extra conditions to make N is injective Hull of M.

In general, I know it is not true, but it seems that it gives a approximation of "functor":M--->E(M)