Andrew, thanks for the article, and the more-to-come.

When you get to data migrations, you might want to cover a problem I ran 
into with them: when I had a data migration in the middle of a sequence of 
other migrations, and then later wanted to set up a new environment, which 
meant running all of the migrations in sequence from the beginning, the 
data migration was now running in the latest models.py code, not the code 
in which it initially worked. So it did not work any more.

In particular, the data migration was a precursor to another schema 
migration were a field would be removed. The data migration was to move the 
existing data in the field-to-be-removed to a different field. So when the 
data migration ran in the longer sequence, the field it was trying to move 
data from was now gone from the code. 

I suspect there are a lot of other ways to shoot yourself in the foot with 
data migrations, so we have stopped using them and just use our own data 
migration scripts outside of the sequence of schema migrations.

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