Convert query builder conditions to MongoDB operations including nested array of subdocuments

I am building an application in Angular 8 on the client side and NodeJS 12 with MongoDB 4 / Mongoose 5 on the server side. I have a query generated by the Angular2 query builder module. The Angular query builder object is sent to the server.

I have a server-side controller function that converts the Angular query object to MongoDB operations. This is working perfectly for generating queries for top-level properties such as RecordID and RecordType. This is also working for building nested and/or conditions.

However, I need to also support querying an array of subdocuments (the "Items" array in the example schema).

Schema

Here is the example schema I am trying to query:

{
  RecordID: 123,
  RecordType: "Item",
  Items: [
    {
      Title: "Example Title 1",
      Description: "A description 1"
    },
    {
      Title: "Example 2",
      Description: "A description 2"
    },
    {
      Title: "A title 3",
      Description: "A description 3"
    },
  ]
}

Working example

Top-level properties only

Here's an example of the query builder output with and/or conditions on top-level properties only:

{ "condition": "or", "rules": [ { "field": "RecordID", "operator": "=", "value": 1 }, { "condition": "and", "rules": [ { "field": "RecordType", "operator": "=", "value": "Item" } ] } ] }

Here's the query builder output after it has been converted to MongoDB operations on top-level properties only:

{ '$expr': { '$or': [ { '$eq': [ '$RecordID', 1 ] }, { '$and': [ { '$eq': [ '$RecordType', 'Item' ] } ] } ] }}

that converts the angular query object to mongodb operators.

Here is the existing query conversion function that

const conditions = { "and": "$and", "or": "$or" };
const operators = { "=": "$eq", "!=": "$ne", "<": "$lt", "<=": "$lte", ">": "$gt", ">=": "$gte" };

const mapRule = rule => ({
    [operators[rule.operator]]: [ "$"+rule.field, rule.value ]
});

const mapRuleSet = ruleSet => {
    return {
        [conditions[ruleSet.condition]]: ruleSet.rules.map(
            rule => rule.operator ? mapRule(rule) : mapRuleSet(rule)
        )
    }
};

let mongoDbQuery = { $expr: mapRuleSet(q) };
console.log(mongoDbQuery);

Issue

The function works for top-level properties only such as RecordID and RecordType, but I need to extend it to support the Items array of subdocuments.

Apparently, to query properties in nested arrays of subdocuments, the $elemMatch operator must be used, based on this related question. However, in my case, the $expr is necessary to build the nested and/or conditions so I can't simply switch to $elemMatch.

QUESTION

How can I extend the query conversion function to also support $elemMatch to query arrays of subdocuments? Is there a way to get the $expr to work?

UI query builder

Here is the UI query builder with the nested "Items" array of subdocuments. In this example, the results should match RecordType equals "Item" AND Items.Title equals "Example Title 1" OR Items.Title contains "Example".

Here is the output generated by the UI query builder. Note: The field and operator property values are configurable.

{"condition":"and","rules":[{"field":"RecordType","operator":"=","value":"Item"},{"condition":"or","rules":[{"field":"Items.Title","operator":"=","value":"Example Title 1"},{"field":"Items.Title","operator":"contains","value":"Example"}]}]}

UPDATE: I may have found a query format that works with the nested and/or conditions with the $elemMatch as well. I had to remove the $expr operator since $elemMatch does not work inside of expressions. I took inspiration from the answer to this similar question.

This is the query that is working. The next step will be for me to figure out how to adjust the query builder conversion function to create the query.

{
  "$and": [{
      "RecordType": {
        "$eq": "Item"
      }
    },
    {
      "$or": [{
          "RecordID": {
            "$eq": 1
          }
        },
        {
          "Items": {
            "$elemMatch": {
              "Title": { "$eq": "Example Title 1" }
            }
          }
        }
      ]
    }
  ]
}