// Plan and execute Convex schema migrations safely, including adding fields, creating tables, and data transformations. Use when schema changes affect existing data.
| name | migration-helper |
| description | Plan and execute Convex schema migrations safely, including adding fields, creating tables, and data transformations. Use when schema changes affect existing data. |
Safely migrate Convex schemas and data when making breaking changes.
Solution: Add as optional first, backfill data, then make required.
// Step 1: Add as optional
users: defineTable({
name: v.string(),
email: v.optional(v.string()),
})
// Step 2: Create migration
export const backfillEmails = internalMutation({
args: {},
handler: async (ctx) => {
const users = await ctx.db.query("users").collect();
for (const user of users) {
if (!user.email) {
await ctx.db.patch(user._id, {
email: `user-${user._id}@example.com`,
});
}
}
},
});
// Step 3: Run migration via dashboard or CLI
// npx convex run migrations:backfillEmails
// Step 4: Make field required (after all data migrated)
users: defineTable({
name: v.string(),
email: v.string(),
})
// Step 1: Add new field (optional)
// Step 2: Copy data with internalMutation
// Step 3: Update schema (remove old field)
// Step 4: Update all code to use new field name
For large tables, process in batches:
export const migrateBatch = internalMutation({
args: {
cursor: v.optional(v.string()),
batchSize: v.number(),
},
handler: async (ctx, args) => {
const items = await ctx.db.query("largeTable").take(args.batchSize);
for (const item of items) {
await ctx.db.patch(item._id, { /* migration logic */ });
}
return {
processed: items.length,
hasMore: items.length === args.batchSize,
};
},
});
For zero-downtime migrations, write to both old and new structure during transition.
Use cron jobs for gradual migration:
import { cronJobs } from "convex/server";
import { internal } from "./_generated/api";
const crons = cronJobs();
crons.interval(
"migrate-batch",
{ minutes: 5 },
internal.migrations.migrateBatch,
{ batchSize: 100 }
);
export default crons;
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