overrideProvider(token) not working in nestJS for Test.createTestingModule - node.js

Question

I'm having a problem with overriding a provider in a test context. I know I'm missing something blindingly obvious, but I've tried loads of permutations and none have worked, so I'm putting this out to the hive mind.
I have the following structure:
shared.module.ts
#Module({
providers: [
DatabaseProvider,
ConfigService,
CacheService
],
exports: [
DatabaseProvider,
ConfigService
CacheService
],
imports: [HttpModule],
})
where DatabaseProvider is defined as:
const DatabaseProvider = {
provide: "MongoConnection",
inject: [ConfigService],
useFactory: async (config: ConfigService): Promise < typeof mongoose | Object > => {
return await mongoose.connect(config.getString("MONGO_URI"), { useNewUrlParser: true });
}
};
my.module.ts
#Module({
imports: [SharedModule, HttpModule],
exports: [MyService],
controllers: [MyController],
providers: [MyService],
})
app.module.ts
#Module({
imports: [
SharedModule,
MyModule
],
providers: [
...
],
controllers: [
...
]
})
my.service.ts
export class MyService {
constructor(
#Inject("MongoConnection") private readonly dbConnection: Connection,
private _cache: CacheService
) { }
...
}
my.spec.ts
let cacheService = {...};
this.db = {...};
const module = await Test.createTestingModule({
imports: [SharedModule, MyModule, HttpModule]
})
.overrideProvider(CacheService)
.useValue(cacheService)
.overrideProvider("MongoConnection")
.useValue(this.db)
.compile();
My issue is that the CacheService appears to be being overridden (it's using redis-mock defined in cacheService, instead of the default redis implementation in CacheService), but the MongoConnection is not - it is still calling the original DatabaseProvider from shared.module.ts.
Can someone point out what I'm missing?