MongoError: Too many fields

Question

Hi,  
We are migrating from MongoDB to DocumentDB and a problem has arisen: "MongoError: Too many fields". Aggregate works fine in MongoDB, but in DocumentDB it raised this error. So, what is the field limit to select/consider in an aggregate in DocumentDB?

Accepted Answer

I was unable to get the `$project` or `$addFields` stages to work with more than 50 fields and could not find any documentation surrounding this limit.

However, at least in the case of `$project`, it appears you can work around the limitation  by combining multiple `$addFields` stages followed by a `$replaceRoot` stage:
```javascript
db.mytable.aggregate([
  {
    $addFields: {
      out: {
        /** fields 1-50 */
      },
    },
  },
  {
    $addFields: {
      out: {
        /** fields 51-100 */
      },
    },
  },
  {
    $addFields: {
      out: {
        /** fields 101-150 */
      },
    },
  },
  {
    $replaceRoot: {
      newRoot: "$out",
    },
  },
]);
```

Answer

Hi. You'll need to identify which actual stage in the aggregation pipeline is triggering the error. I've seen this error before on the $group stage, when there are too many fields to group by, there's a limit of 50 group fields.

MongoError: Too many fields

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