How to mock module which depends implicitly per test in Jest?

I have an integration test where I make actual DB calls to the MongoDB database. But in order to test whether the transaction is expired or not, I need to mock the DB for that particular test. There are many reasons for me to make the actual DB call, I'm mentioning the state just for the sake of this example.

Jest has jest.doMock function but that is helpful only when I wanted to import the function within the test but in my case, It's the DB function which I wanted to mock for that particular test when is getting called inside the express middleware.

There is another option to mock the entire ../db module but that will complicate the tests a lot in my actual project. It would be very easy for me if I can mock the DB call for a specific test and for rest all the tests it should make the real DB calls.

Is there a way to do it in Jest?

// a.ts
import express from "express"
import db from "../db";

const app = express()

app.get("/api/deduct-balance/:txn_id", (req, res) => {
  const txn = await db.findById(txn_id)
  
  // return error message if txn expired
  if (txn.exipre_at <= new Date()) {
    return res.status(401).json({ error: "txn expired" });
  }

  // otherwise update the txn state
  txn.state = "DEDUCTED";
  await txn.save()

  return res.status(200).json();
});

// a.test.ts
import db from "../db";

describe("mixed tests", () => {
  test("should make REAL db calls", async () => {
    await axios.get("/api/deduct-balance/123")
    const txn = await db.findById("123");
    expect(txn.state).toBe("DEDUCTED");
  });

  test("should use MOCKED value", async () => {
    // need a way to mock the DB call so that I can return an expired transaction
    // when I hit the API

    const { data } = await axios.get("/api/deduct-balance/123")
    
    expect(data).toBe({
      error: {
        message: "txn expired"
      }
    });
  });
})