add type def for Aggreaget#model without arguments

[dmshvetsov]

Summary

add missing type definition for Aggregate.prototype.model function that can be used for set or get Model from an Aggregate object.

According to mongoose documentation Aggregate.prototype.model can be used in two ways:

      const aggregate = MyModel.aggregate([{ $match: { answer: 42 } }]);
      aggregate.model() === MyModel; // true
 
      // Change the model. There's rarely any reason to do this.
      aggregate.model(SomeOtherModel);
      aggregate.model() === SomeOtherModel; // true

...to get the aggregation object model and to set it.

But types/aggregate.d.ts is missing type definition for the get case of Aggregate.prototype.model function

This PR aims to fix this.

Examples

[AbdelrahmanHafez]

Welcome @dmshvetsov 👋

Can you please add a test to test/types to prevent future regressions?

Thanks!

[dmshvetsov]

Welcome @dmshvetsov 👋

Can you please add a test to test/types to prevent future regressions?

Thanks!

sure, will do

[dmshvetsov]

@AbdelrahmanHafez could you take another look? I've added type tests for Aggregate.prototype.model

[AbdelrahmanHafez]

LGTM, thanks! 👍