Good app for marking up classes and database relationships

Hi

I am after a good app (preferably one I can download on my computer and not have online) to mark up relationships between classes and database relationships for when i want to prepare for a project. I am a Ruby/rails/javascript/web developer.

I am looking at Creately and Figma at the moment but would love some more suggestions, and why you think your choice is best suited for software developers.

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Thanks Stephan