nextjs - 💡(How to fix) Fix Mongoose populate fails on first API request in Next.js: MissingSchemaError for referenced model [1 comments, 2 participants]

Link to the code that reproduces this issue

N/A — this issue is due to Mongoose model registration in Next.js API routes, not a Next.js bug. See code snippets below for reproduction.

To Reproduce

. Create two Mongoose models: Product and Category. 2. In Product schema, add a reference: category: { type: Schema.Types.ObjectId, ref: "Category" }. 3. Create a Next.js API route that fetches products with populate("category"). 4. Start the Next.js dev server. 5. Make the first request to the Product API route. 6. Observe the error: MissingSchemaError: Schema hasn't been registered for model "Category".

Current vs. Expected behavior

Expected behavior: Populate works on the first request and returns category details.

Actual behavior: First request fails with: MissingSchemaError: Schema hasn't been registered for model "Category". Subsequent requests work fine if the Category model has been hit/loaded.

Provide environment information

Operating System: macOS 14.6
Node.js Version: 20.17.0
npm Version: 10.8.2
pnpm Version: 9.10.0
Next.js Version: 15.0.0-canary.148

Which area(s) are affected? (Select all that apply)

Error Overlay

Which stage(s) are affected? (Select all that apply)

next dev (local)

Additional context

This issue occurs because Mongoose requires referenced models to be imported before calling populate(). In Next.js API routes or server actions, if the referenced model (e.g., Category) hasn't been imported yet, the first request fails. Solution: create a central models loader (models/index.ts) and import all models before dbConnect in API routes.