convex-migrations

What this skill does

# Convex Migrations

Evolve your Convex database schema safely with patterns for adding fields, backfilling data, removing deprecated fields, and maintaining zero-downtime deployments.

## Documentation Sources

Before implementing, do not assume; fetch the latest documentation:

- Primary: https://docs.convex.dev/database/schemas
- Schema Overview: https://docs.convex.dev/database
- Migration Patterns: https://stack.convex.dev/migrate-data-postgres-to-convex
- For broader context: https://docs.convex.dev/llms.txt

## Instructions

### Migration Philosophy

Convex handles schema evolution differently than traditional databases:

- No explicit migration files or commands
- Schema changes deploy instantly with `npx convex dev`
- Existing data is not automatically transformed
- Use optional fields and backfill mutations for safe migrations

### Adding New Fields

Start with optional fields, then backfill:

```typescript
// Step 1: Add optional field to schema
// convex/schema.ts
import { defineSchema, defineTable } from "convex/server";
import { v } from "convex/values";

export default defineSchema({
  users: defineTable({
    name: v.string(),
    email: v.string(),
    // New field - start as optional
    avatarUrl: v.optional(v.string()),
  }),
});
```

```typescript
// Step 2: Update code to handle both cases
// convex/users.ts
import { query } from "./_generated/server";
import { v } from "convex/values";

export const getUser = query({
  args: { userId: v.id("users") },
  returns: v.union(
    v.object({
      _id: v.id("users"),
      name: v.string(),
      email: v.string(),
      avatarUrl: v.union(v.string(), v.null()),
    }),
    v.null()
  ),
  handler: async (ctx, args) => {
    const user = await ctx.db.get(args.userId);
    if (!user) return null;

    return {
      _id: user._id,
      name: user.name,
      email: user.email,
      // Handle missing field gracefully
      avatarUrl: user.avatarUrl ?? null,
    };
  },
});
```

```typescript
// Step 3: Backfill existing documents
// 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.optional(v.string()),
  },
  returns: v.object({
    processed: v.number(),
    hasMore: v.boolean(),
  }),
  handler: async (ctx, args) => {
    const result = await ctx.db
      .query("users")
      .paginate({ numItems: BATCH_SIZE, cursor: args.cursor ?? null });

    let processed = 0;
    for (const user of result.page) {
      // Only update if field is missing
      if (user.avatarUrl === undefined) {
        await ctx.db.patch(user._id, {
          avatarUrl: generateDefaultAvatar(user.name),
        });
        processed++;
      }
    }

    // Schedule next batch if needed
    if (!result.isDone) {
      await ctx.scheduler.runAfter(0, internal.migrations.backfillAvatarUrl, {
        cursor: result.continueCursor,
      });
    }

    return {
      processed,
      hasMore: !result.isDone,
    };
  },
});

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

```typescript
// Step 4: After backfill completes, make field required
// convex/schema.ts
export default defineSchema({
  users: defineTable({
    name: v.string(),
    email: v.string(),
    avatarUrl: v.string(), // Now required
  }),
});
```

### Removing Fields

Remove field usage before removing from schema:

```typescript
// Step 1: Stop using the field in queries and mutations
// Mark as deprecated in code comments

// Step 2: Remove field from schema (make optional first if needed)
// convex/schema.ts
export default defineSchema({
  posts: defineTable({
    title: v.string(),
    content: v.string(),
    authorId: v.id("users"),
    // legacyField: v.optional(v.string()), // Remove this line
  }),
});

// Step 3: Optionally clean up existing data
// convex/migrations.ts
export const removeDeprecatedField = internalMutation({
  args: {
    cursor: v.optional(v.string()),
  },
  returns: v.null(),
  handler: async (ctx, args) => {
    const result = await ctx.db
      .query("posts")
      .paginate({ numItems: 100, cursor: args.cursor ?? null });

    for (const post of result.page) {
      // Use replace to remove the field entirely
      const { legacyField, ...rest } = post as typeof post & { legacyField?: string };
      if (legacyField !== undefined) {
        await ctx.db.replace(post._id, rest);
      }
    }

    if (!result.isDone) {
      await ctx.scheduler.runAfter(0, internal.migrations.removeDeprecatedField, {
        cursor: result.continueCursor,
      });
    }

    return null;
  },
});
```

### Renaming Fields

Renaming requires copying data to new field, then removing old:

```typescript
// Step 1: Add new field as optional
// convex/schema.ts
export default defineSchema({
  users: defineTable({
    userName: v.string(), // Old field
    displayName: v.optional(v.string()), // New field
  }),
});

// Step 2: Update code to read from new field with fallback
export const getUser = query({
  args: { userId: v.id("users") },
  returns: v.object({
    _id: v.id("users"),
    displayName: v.string(),
  }),
  handler: async (ctx, args) => {
    const user = await ctx.db.get(args.userId);
    if (!user) throw new Error("User not found");

    return {
      _id: user._id,
      // Read new field, fall back to old
      displayName: user.displayName ?? user.userName,
    };
  },
});

// Step 3: Backfill to copy data
export const backfillDisplayName = internalMutation({
  args: { cursor: v.optional(v.string()) },
  returns: v.null(),
  handler: async (ctx, args) => {
    const result = await ctx.db
      .query("users")
      .paginate({ numItems: 100, cursor: args.cursor ?? null });

    for (const user of result.page) {
      if (user.displayName === undefined) {
        await ctx.db.patch(user._id, {
          displayName: user.userName,
        });
      }
    }

    if (!result.isDone) {
      await ctx.scheduler.runAfter(0, internal.migrations.backfillDisplayName, {
        cursor: result.continueCursor,
      });
    }

    return null;
  },
});

// Step 4: After backfill, update schema to make new field required
// and remove old field
export default defineSchema({
  users: defineTable({
    // userName removed
    displayName: v.string(),
  }),
});
```

### Adding Indexes

Add indexes before using them in queries:

```typescript
// Step 1: Add index to schema
// convex/schema.ts
export default defineSchema({
  posts: defineTable({
    title: v.string(),
    authorId: v.id("users"),
    publishedAt: v.optional(v.number()),
    status: v.string(),
  })
    .index("by_author", ["authorId"])
    // New index
    .index("by_status_and_published", ["status", "publishedAt"]),
});

// Step 2: Deploy schema change
// Run: npx convex dev

// Step 3: Now use the index in queries
export const getPublishedPosts = query({
  args: {},
  returns: v.array(v.object({
    _id: v.id("posts"),
    title: v.string(),
    publishedAt: v.number(),
  })),
  handler: async (ctx) => {
    const posts = await ctx.db
      .query("posts")
      .withIndex("by_status_and_published", (q) =>
        q.eq("status", "published")
      )
      .order("desc")
      .take(10);

    return posts
      .filter((p) => p.publishedAt !== undefined)
      .map((p) => ({
        _id: p._id,
        title: p.title,
        publishedAt: p.publishedAt!,
      }));
  },
});
```

### Changing Field Types

Type changes require careful migration:

```typescript
// Example: Change from string to number for a "priority" field

// Step 1: Add new field with new type
// convex/schema.ts
export default defineSchema({
  tasks: defineTable({
    title: v.string(),
    priority: v.string(), // Old: "low", "medium", "high"
    priorityLevel: v.optional(v.number()), // N