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Comment by tomhoule | Hacker News Reader

Yep we looked at Django ORM as an inspiration. I unfortunately don't have the bandwidth right now to write a lengthy thoughtful response, but quickly on a few points:

- The replaying of the migrations history is exactly what we do, but not in-memory, rather directly on a temporary (shadow) database. It's a tradeoff, but it lets us be a lot more accurate and be more accurate, since we know exactly what migrations will do, rather than guessing from an abstract representation outside of the db.

- I wrote a message on the why of no down migrations above. It's temporary — we want something at least as good, which may be just (optional) down migrations.

- The discrepancy resolving in our case is mainly about detecting that schemas don't match, and how, rather than actually migrating them (we recommend hard resets + seeding in a lot of cases in development), so data is not as much of an issue.

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Well, of course I don't know about the internals, but having used Django migrations for a decade now (it used to be a standalone solution called "South" back then), I haven't really run into any inaccuracies and can't really imagine how those could happen. As far as I can see, the main difference is that they are storing and intermediate format (that they can map to SQL unambigously) while you immediately generate the SQL.

Django doesn't try (too hard) to validate your model against the actual DB schema. Because why would it? You either ran all the migrations and then it matches or you didn't and then you have to. (Unless you write your own migrations and screw them up. But that's rare and you can catch it with testing.) While your focus then seems to be to check if the schema (whatever is there in the db) matches the model definition. Based on my experience (as a user) this latter is not really something that I need help with.

Data is actually an issue in development and hard resets + (re)seeding is pretty inconvenient compared to what django provides. E.g. in my current project we're using a db snapshot that we've pulled from production about two years ago (after thorough anonymization, of course). We initialize new dev environments and then it gets migrated. It probably takes about half a minute to run as opposed to about 2 seconds of back migrating 2-3 steps.

It makes a lot of sense. I have a fair amount of Rails experience with ActiveRecord, and it was also my impression that the database schema drifting in development is rarely a problem, but I now think it's a bit of a fuzzy feeling and discrepancies definitely sneak in. The main sources of drift in development would be 1. switching branches, and more generally version control with collaborators, 2. iterating on/editing of migrations, 3. manual fiddling with the database

One assumption with Prisma Migrate is that since we are an abstraction of the database, and support many of them, we'll never cover 100% of the features (e.g. check constraints, triggers, stored procedures, etc.), so we have to take the database as the source of truth and let users define what we don't represent in the Prisma Schema. On SQL databases, we let you write raw SQL migrations for example, so you have full control if you need it.

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