$not operator does not handle non-array values correctly.

[redgeoff]

I'd like to start by saying that your library looks awesome!

I'm considering making mingo the MongoDB query engine for MSON as it implements aggregation, a very needed concept that is currently missing.

My hold up is that in my quick tests, it looks like there may be a problem with basic operators like $not. For example, take a look at this codsandbox and you'll see that the output of bar is true and not the negation of foo. In other words, shouldn't bar=false?

For convenience, here is the code:

import { Aggregator } from "mingo/aggregator";
import "mingo/init/system";

const collection = [
  {
    foo: true
  }
];

let agg = new Aggregator([
  {
    $project: {
      bar: {
        $not: "$foo"
      }
    }
  }
]);

let result = agg.run(collection);
console.log(result);

By comparison, this is how MongoDB handles this same aggregation. Please note that the Result has bar=false

Configuration:

[
  {
    foo: true
  }
]

Query:

db.collection.aggregate([
  {
    $project: {
      bar: {
        $not: "$foo"
      }
    }
  }
])

Result:

[
  {
    "_id": ObjectId("5a934e000102030405000000"),
    "bar": false
  }
]

[kofrasa]

Thanks and I appreciate you considering this library.

On the issue itself, the $not operator is actually implemented although it seems there is an inconsistency with MongoDB.

The documentation does say the value for the expression when used in an aggregation context must be [ <expression> ], that is, it must be wrapped in an array. If you changed your example query to bar: { $not: [ "$foo" ] }, you will get the expected result. See example

Nonetheless given the inconsistency it is fair to treat this a bug. The documentation alone does not constitute a spec of any kind.

[redgeoff]

Awesome, thanks for the clarification! I'll leave this issue open to leave a record of the bug.