Migration Helper

# Convex Migration Helper

Safely migrate Convex schemas and data when making breaking changes.

## When to Use

- Adding new required fields to existing tables
- Changing field types or structure
- Splitting or merging tables
- Renaming fields
- Migrating from nested to relational data

## Migration Principles

1. **No Automatic Migrations**: Convex doesn't automatically migrate data
2. **Additive Changes are Safe**: Adding optional fields or new tables is safe
3. **Breaking Changes Need Code**: Required fields, type changes need migration code
4. **Zero-Downtime**: Write migrations to keep app running during migration

## Safe Changes (No Migration Needed)

### Adding Optional Field

```typescript
// Before
users: defineTable({
  name: v.string(),
})

// After - Safe! New field is optional
users: defineTable({
  name: v.string(),
  bio: v.optional(v.string()),
})
```

### Adding New Table

```typescript
// Safe to add completely new tables
posts: defineTable({
  userId: v.id("users"),
  title: v.string(),
}).index("by_user", ["userId"])
```

### Adding Index

```typescript
// Safe to add indexes at any time
users: defineTable({
  name: v.string(),
  email: v.string(),
})
  .index("by_email", ["email"]) // New index
```

## Breaking Changes (Migration Required)

### Adding Required Field

**Problem**: Existing documents won't have the new field.

**Solution**: Add as optional first, backfill data, then make required.

```typescript
// Step 1: Add as optional
users: defineTable({
  name: v.string(),
  email: v.optional(v.string()), // Start optional
})

// Step 2: Create migration
import { internalMutation } from "./_generated/server";
import { v } from "convex/values";

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`, // Default value
        });
      }
    }
  },
});

// 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(), // Now required
})
```

### Changing Field Type

**Example**: Change `tags: v.array(v.string())` to separate table

```typescript
// Step 1: Create new structure (additive)
tags: defineTable({
  name: v.string(),
}).index("by_name", ["name"]),

postTags: defineTable({
  postId: v.id("posts"),
  tagId: v.id("tags"),
})
  .index("by_post", ["postId"])
  .index("by_tag", ["tagId"]),

// Keep old field as optional during migration
posts: defineTable({
  title: v.string(),
  tags: v.optional(v.array(v.string())), // Keep temporarily
})

// Step 2: Write migration
export const migrateTags = internalMutation({
  args: { batchSize: v.optional(v.number()) },
  handler: async (ctx, args) => {
    const batchSize = args.batchSize ?? 100;

    const posts = await ctx.db
      .query("posts")
      .filter(q => q.neq(q.field("tags"), undefined))
      .take(batchSize);

    for (const post of posts) {
      if (!post.tags || post.tags.length === 0) {
        await ctx.db.patch(post._id, { tags: undefined });
        continue;
      }

      // Create tags and relationships
      for (const tagName of post.tags) {
        // Get or create tag
        let tag = await ctx.db
          .query("tags")
          .withIndex("by_name", q => q.eq("name", tagName))
          .unique();

        if (!tag) {
          const tagId = await ctx.db.insert("tags", { name: tagName });
          tag = { _id: tagId, name: tagName };
        }

        // Create relationship
        const existing = await ctx.db
          .query("postTags")