/convex-migrations

Convex migrations

Produces a schema change plus a batched backfill that keeps every deploy green. The one rule: the schema must describe the documents that exist right now, not the documents you want. Change the data first, then tighten the validator.

Convex has no migration files or migrate command. npx convex dev pushes the schema and validates every existing document against it. Existing data is never transformed for you.

When to reach for this

• Adding a field that should be required, but the table already has rows
• Renaming a field, changing its type, or removing it
• Splitting one table into two or merging two into one
• npx convex dev fails with Schema validation failed after a schema edit
• A backfill needs to touch more rows than one mutation can handle

For step by step recipes (rename, change type, split, merge, add required, remove) open references/migration-patterns.md. For installing and running @convex-dev/migrations open references/migrations-component.md.

The safe sequence

Every migration follows the same six steps. Skipping one is how deploys break.

1. Make the field optional in convex/schema.ts (or add the new field as v.optional).
2. Deploy with npx convex dev. Update readers to handle undefined and writers to set the new shape.
3. Backfill existing documents in batches with an internal mutation or the migrations component.
4. Flip the validator to its final shape (v.string() instead of v.optional(v.string()), or drop the old field).
5. Deploy again. Validation now passes because every document already matches.
6. Clean up: delete fallback code, remove the backfill function, drop stale indexes.

Steps 1 and 2 must ship before step 3 starts. Otherwise new writes keep producing old shaped documents while the backfill runs.

Hand rolled batched backfill

One internalMutation that pages through the table with paginate, patches only documents that still need it, then reschedules itself with the continue cursor. Each batch is its own transaction, so a large table never hits the per function limit.

// convex/migrations.ts
import { internalMutation } from "./_generated/server";
import { internal } from "./_generated/api";
import { v } from "convex/values";

const BATCH_SIZE = 100;

export const backfillAvatarUrl = internalMutation({
  args: { cursor: v.union(v.string(), v.null()) },
  returns: v.null(),
  handler: async (ctx, args) => {
    const result = await ctx.db
      .query("users")
      .paginate({ numItems: BATCH_SIZE, cursor: args.cursor });

    for (const user of result.page) {
      // Idempotent: skip documents that already have the field
      if (user.avatarUrl === undefined) {
        await ctx.db.patch(user._id, {
          avatarUrl: defaultAvatar(user.name),
        });
      }
    }

    // Reschedule with the cursor until the table is exhausted
    if (!result.isDone) {
      await ctx.scheduler.runAfter(0, internal.migrations.backfillAvatarUrl, {
        cursor: result.continueCursor,
      });
    }
    return null;
  },
});

function defaultAvatar(name: string): string {
  return `https://api.dicebear.com/7.x/initials/svg?seed=${encodeURIComponent(name)}`;
}

Start it from the CLI against the dev deployment:

npx convex run migrations:backfillAvatarUrl '{"cursor": null}'

Keep the mutation idempotent. Re running it after a partial failure must be safe. Use internalMutation, never a public mutation, so it cannot be called from a client. Use ctx.db.patch for adding or changing fields and ctx.db.patch(id, { field: undefined }) to remove one.

When to use the component instead

@convex-dev/migrations wraps the pattern above and adds what the hand rolled version lacks: persisted progress per migration, status checks, dry runs, cancel, and ordered runs of several migrations. Reach for it when:

• The project will run more than one or two migrations over its life
• You need to know whether a migration already completed in production
• You want a dry run before touching real data
• Several migrations must run in a fixed order

Minimal setup:

// convex/convex.config.ts
import { defineApp } from "convex/server";
import migrations from "@convex-dev/migrations/convex.config.js";

const app = defineApp();
app.use(migrations);
export default app;

// convex/migrations.ts
import { Migrations } from "@convex-dev/migrations";
import { components } from "./_generated/api";
import { DataModel } from "./_generated/dataModel";

export const migrations = new Migrations<DataModel>(components.migrations);

export const addDefaultRole = migrations.define({
  table: "users",
  migrateOne: async (ctx, user) => {
    if (user.role === undefined) {
      await ctx.db.patch(user._id, { role: "user" });
    }
  },
});

npx convex run migrations:addDefaultRole '{"dryRun": true}'
npx convex run migrations:addDefaultRole

A one off backfill on a small table does not need the component. The hand rolled mutation is fine.

Reading the deploy time schema error

When a schema push fails, npx convex dev prints the table, one offending document id, and the field that does not match:

Schema validation failed
Document with ID "j57abc..." in table "users" does not match the schema:
Object is missing the required field `avatarUrl`.
Consider wrapping the field validator in `v.optional(...)` if this is expected.

Read it as: existing data in users predates the field. The fix is never to delete the document. Wrap the field in v.optional, deploy, backfill, then remove the v.optional. If the error says the field has the wrong type (for example string where number is expected), widen the validator to v.union(v.string(), v.number()), backfill the conversion, then narrow it.

Do not pass { schemaValidation: false } to defineSchema to make the push succeed. It hides the mismatch and moves the failure into your queries.

Common mistakes

Checklist

• New or changed field is v.optional (or a widened v.union) in the first deploy
• Readers handle undefined and writers produce the new shape before the backfill starts
• Backfill is an internalMutation or migrations.define, never public
• Backfill pages with paginate and reschedules via ctx.scheduler.runAfter(0, internal....)
• Each patch is guarded so re running is safe
• Backfill ran to completion on dev before touching prod
• Validator flipped to its final shape and deployed without a validation error
• Fallback code, old field, and backfill function removed
• Indexes that referenced the old field are dropped or renamed

Docs

• https://docs.convex.dev/llms.txt
• https://docs.convex.dev/database/schemas
• https://docs.convex.dev/database/pagination
• https://www.convex.dev/components/migrations
• https://stack.convex.dev/migrating-data-with-mutations