I am currently writing a thesis and need to display the schema of my MongoDB in a diagram. I have found no resources about diagrams for document-based databases.
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We found class diagrams to actually be one of the best ways to represent a mongo schema design.
It can capture most of the items that a document will have such as arrays, embedded objects and even references.
General guidelines we use to relate onto concepts to uml
Embed = Composition aggregation
Reference = Association class
If you're unfamiliar with the uml terminology then this is a decent intro.
UML intro from IBM site