When you're starting with a blank slate of a project codebase, having to write and then keep evolving a SQL schema -- and do data migrations -- can be a real pa

When you're starting with a blank slate of a project codebase, having to write and then keep evolving a SQL schema -- and do data migrations -- can be a real pain. With MongoDB, a collection (think table) has no schema, so each object (document, approx like a row or set of related rows) can have whatever format you want, can vary from object to object. While there is greater risk, esp in long run, of accumulating objects with structures that are unexpected or unsupported by dependent applications, in the short run it gives you a development speed boost. Also, the native document format is JSON, which, if you're doing web development, is more likely to be the web client-side data format you'd want to deal with anyway. Plus JSON is very close to a Python dict, which makes Pythonistas happy on the server-side. JSON is also a bit more of a "self-documenting" format compared to say what a SQL result dump looks like (every column/property has it's name/key declared in the dump itself). Also, if you have model data that's naturally document-like, with many sub-structures within it, hierarchically composed, then rather than having to slice it up across many distinct table schemas, each with an appropriate chain of foreign key references leading back to the mother table, you instead just stuff the entire model document, including all it's sub-structures, into a single slot in the Mongo database. So it simplifies that as well. I recently evaluated a lot of different database systems too for a new project, and settled on MongoDB for a key part of it. While it has many positives (the above, plus performance and scalability) one of the negatives that worries me a little bit is the "running with scissors" feeling of not having database-enforced table schemas. Ultimately, we'll find out firsthand whether that leads to disaster or not, and whether it's a net win. My bet now is net win.