How to cite in LaTeX using Mendeley?

Another simple way is to select the file in Mendeley list, then right-click and select "copy as", then "LaTex citation command" (which corresponds to ctrl+K), and last paste the thing inside the brackets of \cite{}

You have several options here. One is, as you suggest, to simply keep the .bib file open in the text editor and search for the cite key as you need it. Another is to use some software such as JabRef or BibDesk to manage the .bib file, which will be somewhat complicated since Mendeley overwrites the auto-generated .bib file every time a reference is updated.

Thus, I prefer to use the better searching capabilities of Mendeley, so I usually look up the citation key there. Unfortunately, Mendeley have decided that citation keys should not be visible by default, so you have to turn them on. Here I'll copy from my answer to a different question: Go to the menu->Tools->Options->Document Details, then choose the document type (for instance, Journal Article), then check the Citation Key box. Then, a field for Citation Key should be visible in the details pane on the right hand side of the window.