Add unit tests for sample 06-mongoose

[martinvysnovsky]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Improvements in samples

What is the current behavior?

Sample 06-mongoose is missing some unit tests.

Issue Number: N/A

What is the new behavior?

More unit tests in sample 06-mongoose.

Does this PR introduce a breaking change?

• Yes
• No

Other information

[coveralls]

Pull Request Test Coverage Report for Build 2d502500-1db4-4ec5-b760-8610b6ab6adf

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 92.213%

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Totals
Change from base Build 71551605-c356-4a76-8f46-8de2acf07906:	0.0%
Covered Lines:	6750
Relevant Lines:	7320

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💛 - Coveralls

[martinvysnovsky]

I would like to include E2E tests too, but I need to use a database for that. Usually, I am using testcontainers in my E2E tests. Do you agree to use additional dependency (testcontainers) here?

(also for unit tests of services I do not like to mock the model. Usually, in a real codebase, you have to improve these mocks every time you change the service. I think, using a real database and actually run the queries give you more confidence in your tests. Previously I did use inmemory mongo db, now I am using testcontainers here too. The result is that these tests are not pure unit tests anymore but rather integration tests. Still, I think it makes more sense for services. So again. If you agree then I can rewrite this example to use testcontainers)