Why Better Auth's MongoDB adapter can't populate() a Mongoose ref

If your Node app already uses Mongoose and you wire up Better Auth with its official MongoDB adapter, this looks fine at first:

import { betterAuth } from "better-auth";
import { mongodbAdapter } from "better-auth/adapters/mongodb";
import mongoose from "mongoose";

const client = mongoose.connection.getClient();

export const auth = betterAuth({
  database: mongodbAdapter(client.db()),
});

Then you try to reference a Better-Auth-created user from your own model:

const Post = mongoose.model(
  "Post",
  new Schema({
    title: String,
    author: { type: Schema.Types.ObjectId, ref: "user" },
  }),
);

const post = await Post.findOne({ title: "..." }).populate("author");

post.author; // null

populate() silently fails. Not an error — just null, like the ref never resolved. If you've hit this, you're not doing anything wrong.

Why it happens

mongodbAdapter() talks to the raw mongodb driver, not Mongoose. It never registers a Mongoose model for user, session, account, or any of Better Auth's own tables. Two consequences fall out of that:

1. You end up with two disconnected views of the same database. Better Auth writes through MongoClient directly. Your own code reads and writes through Mongoose. No shared schema, no shared validation, no shared hooks between the two.
2. The _id types don't line up. Better Auth's default ID generator produces a 32-character base62 string — not a real ObjectId. Mongoose's populate() needs a real ObjectId to resolve a ref against. There's nothing valid on the other end.

This isn't a one-off — it's been an open, documented gap on Better Auth's own repo since February 2025:

• better-auth#1492 — forced to install the raw mongodb package as a duplicate dependency, even in apps that only ever use Mongoose.
• better-auth#6289 — id/session mismatches from mixing Mongoose reads with raw-driver writes on the same collections.
• better-auth discussion #9364 and #1921 — people hitting exactly this populate() failure and asking for a real Mongoose adapter.

The workaround everyone lands on — mongoose.connection.getClient(), hand the raw client to mongodbAdapter() — only fixes where Better Auth connects. It doesn't fix what it stores. Your ref field still has nothing valid to resolve against.

The actual fix

better-auth-mongoose registers Better Auth's tables as real, extendable Mongoose models on a connection you already own, and overrides ID generation to produce real 24-character ObjectId hex strings instead of Better Auth's default base62 IDs — converting between ObjectId and string at the boundary so Better Auth's own core still sees plain strings, but MongoDB stores actual ObjectIds underneath.

import { betterAuth } from "better-auth";
import { mongooseAdapter } from "better-auth-mongoose";
import mongoose, { Schema } from "mongoose";

await mongoose.connect(process.env.MONGO_URI!);

export const auth = betterAuth({
  database: mongooseAdapter(mongoose.connection),
});

Same Post model, same .populate("author") call, no workaround:

const Post = mongoose.model(
  "Post",
  new Schema({
    title: String,
    author: { type: Schema.Types.ObjectId, ref: "user", required: true },
  }),
);

const post = await Post.findOne({ title: "..." }).populate("author").lean().exec();

post.author.email; // resolved

This is a real, CI-run test in the repo, not a claim in a README:

describe("the differentiator: a consumer's own model can .populate() a Better-Auth-created user", () => {
  it("resolves Post.author via .populate() after Better Auth creates the user", async () => {
    const auth = betterAuth({ database: mongooseAdapter(connection) });

    const { user } = await auth.api.signUpEmail({
      body: { email: "[e-mail]", password: "correct-horse-battery-staple", name: "Post Author" },
    });

    await Post.create({ title: "Hello, populate()", author: coerceToObjectId(user.id) });

    const post = await Post.findOne({ title: "Hello, populate()" }).populate("author").lean().exec();

    expect(post.author.email).toBe("[e-mail]"); // passes
  });
});

It also passes the official @better-auth/test-utils adapter contract suite, so it's not just "populate works" — it's a fully conformant Better Auth adapter.

What else comes with it

• Zero direct dependencies. mongoose and better-auth are peers you already have; the raw mongodb driver is never pulled in.
• Schema extension that doesn't fight Better Auth. schemas: { user: new Schema({ role: { type: String, default: "member" } }) } merges your fields in without letting a required Better Auth field get accidentally loosened.
• Transactions on by default, using real Mongoose sessions, with automatic fallback to non-transactional writes on a standalone mongod (common in local dev) instead of crashing on boot.
• Adapter-level joins. Better Auth 1.4+ can push joins down to the adapter for a documented 2–3x latency improvement on endpoints like get-session. This adapter turns that straight into .populate() calls — flip experimental: { joins: true } and it's already wired up.
• If you're on NestJS: examples/nestjs-mongoose is a complete app built on @thallesp/nestjs-better-auth, exercised over real HTTP in CI on every push.
• If you're building multi-tenant SaaS on top of Better Auth's organization plugin, there's a companion package, better-auth-mongoose-tenant, for automatic tenant-scoped queries on your own models.

pnpm add better-auth-mongoose mongoose better-auth

better-auth-mongoose.ashwinsathian.com has the full writeup, a compatibility matrix (Mongoose 6–9, Better Auth 1.4–1.6), and more recipes. Repo's at github.com/AshwinSathian/better-auth-mongoose — MIT licensed, not affiliated with the Better Auth team, built because this gap has sat open for a year and a half.