How to perform numeric comparisons on string fields with units in MongoDB/Mongoose?

Hi everyone,

I’m building a recipe API using Node.js, Express, and Mongoose. My MongoDB documents have nested nutrient fields stored as strings with units, for example:

{
"nutrients": {
"calories": "389 kcal",
"proteinContent": "5 g",
"fatContent": "21 g"
},
"serves": "8 servings",
"title": "Sweet Potato Pie"
}

I want to allow clients to filter recipes by numeric ranges, e.g., calories less than 300 kcal.

Problem:

Since calories is stored as "389 kcal" (string), MongoDB cannot perform $gt, $lt, $gte, $lte operations correctly.

I’m also storing serves as "8 servings" and other nutrients similarly with units.

Questions:

What is the best way to perform numeric comparisons for fields stored as strings with units?

Should I migrate the database to store numeric values and units separately?

If migration isn’t possible, how can I filter recipes efficiently — via aggregation or Node.js post-processing?