More examples in Mongo

[orasik]

Yesterday I struggled to create migrations with some use cases (adding a new field, updating field with a value from another) so I thought of adding these examples as it might help someone and save time :)

[dhui]

Unfortunately, I'm not familiar with MongoDB queries or schema changes, so am not comfortable merging this in without another reviewer that knows MongoDB better.

@zikaeroh @erikdubbelboer If you know MongoDB well enough, could you review this PR?

[erikdubbelboer]

I have never worked with MongoDB.

[orasik]

is there any integration test I can do to help?

[dhui]

We could run the example migrations as part of our tests. e.g. add TestExample(t *testing.T, sourceDir string) to database/testing and update all of the existing DB tests accordingly.

When TestExample() is given empty string or empty directory, it should pass.

[coveralls]

Pull Request Test Coverage Report for Build 609

• 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 51.402%

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Totals
Change from base Build 608:	0.0%
Covered Lines:	2346
Relevant Lines:	4564

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💛 - Coveralls

[dhui]

Turns out TestMigrate() runs migrations using the examples