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I auto-configured Spring Boot REST controllers by adding their names to the spring.factories file. On the other hand, I did not have much luck with auto-configuring Mongo collections (yet).

I then ran the consumer’s codebase and used Postman to hit the newly-offered endpoints. Below is the response seen in Postman:

{
  "error": {
    "IllegalArgumentException": "Couldn't find PersistentEntity for type class yourTLD.yourOrg.yourProject.somePackage.SomeModel!"
  }
}

I thought the Mongo repository was not auto-configured, so I went to check the consumer’s logs.

Finished Spring Data repository scanning in x ms. Found 1 MongoDB repository interfaces.
Finished Spring Data repository scanning in x ms. Found 1 MongoDB repository interfaces.
Finished Spring Data repository scanning in x ms. Found 2 Reactive MongoDB repository interfaces.

I conjectured the first “1 MongoDB repository interfaces” came from the auto-configured repository. When I removed the custom starter from build.gradle, there were only logs for 3 repositories. This behaviour supported my hypothesis.

(You may be wondering why there are reactive Mongo repositories. It’s because we are transitioning from a reactive codebase to a non-reactive one.)

However, there was no other useful information as the consumer did not log any errors. The MongoDB provisioner was a massive platform with a ton of logs. I did not know which one to check and whether I should even check those logs. I proceeded to scrutinise both the consumer and the custom starter’s codebases.

This line piqued my curiosity:

@EnableMongoRepositories(value = "yourTLD.yourOrg.yourProject.somePackageContainingRepositories")

It did not point to the custom starter’s package containing the auto-configured repository. This could be why I received the IllegalArgumentException error.