Example of an Apostrophe migration

index.js

// in lib/modules/pet-owners/index.js

module.exports = {
  extend: 'apostrophe-pieces',
  name: 'pet-owner',
  addFields: [
    {
      name: 'cat',
      label: 'Cat',
      type: 'string'
    }
  ],
  construct(self, options) {
    self.apos.migrations.add('renameFelineField', async () => {
      await self.apos.migrations.eachDoc({
        // This is a raw mongo query, we must take care to leave other piece types alone
        type: 'pet-owner',
        // Don't bother with docs that never had a feline field
        feline: { $exists: 1 }
      }, async (doc) => {
        // Raw mongodb update so we can use $set and $unset and avoid race conditions
        await self.apos.docs.db.updateOne({
          _id: doc._id
        }, {
          $set: {
            cat: doc.feline
          },
          $unset: {
            feline: 1
          }
        });
      });
    });
  };
};