Choosing a database
Answer out loud or in your head first, then compare.
Answer
I assess the shape of the data, typical queries, relations, transactions, scale, consistency requirements and the team's skills.
MongoDB may fit: the data naturally has a document structure, reads cover whole aggregates, and the model evolves with the product.
A relational database may fit: many relationships, strong integrity constraints and extensive cross-cutting reports are key.
In practice: I model a few of the most important operations and measure them on representative data. I also look at backup, restore and costs.
Pitfall: the choice shouldn't come only from the backend being written in JavaScript or documents looking like JSON.
How did it go? Be honest — “Not yet” puts the question on your review list.