Find MongoDB document and only matching array elements w/ C# driver

Question

I'm trying to return a document, and that document should have it's array filtered such that it only contains one item. I've seen many similar questions, but none of them deal with dynamic queries. There may be several constraints so I have to be able to keep adding to the filter.

{
  "_id" : ObjectId("6058f722e9e41a3d243258dc"),
  "fooName" : "foo1",
  "fooCode" : 1,
  "bar" : [
    {
      "barCode" : "123",
      "barName" : "Rick's Cafe",
      "baz" : [
        {
          "bazId" : "00",
          "bazDescription" : "Ilsa"
        },
        {
          "bazId" : "21",
          "bazDescription" : "Victor"
        }
      ]
    },
    {
      "barCode" : "456",
      "barName" : "Draco Tavern",
      "baz" : [
        {
          "bazId" : "00",
          "bazDescription" : "Rick Shumann"
        }
      ]
    }
  ]
}

This is my attempt, it returns a document who's array contains the barCode, and the array's entire contents are included.

Expression<Func<Foo, bool>> filter = x => x.FooCode == 1;

string barCode = "456"
if (!String.IsNullOrEmpty(barCode))
{
  Expression<Func<Foo, bool>> newPred =
    x => x.Bar.Any(s => s.BarCode == barCode);
  filter = filter.CombineAnd(newPred);
}

var fooQuery =
  _fooCollection
    .Find(filter);

How do I remove non-matching array elements, but only if an array element was specified?

BSSchwarzkopf · Accepted Answer · 2021-03-25 16:29:59Z

1

Unwind to convert the single document into a document per nested-array element in the shape of:

{
  "_id" : ObjectId("6058f722e9e41a3d243258dc"),
  "fooName" : "foo1",
  "fooCode" : 1,
  "bar": {
    "barCode" : "123",
    "barName" : "Rick's Cafe",
    ...
  }
}

Match to find the element you want
Group to recombine the nested-array

So the resulting C# might look like:

var fooQuery = _fooCollection.Aggregate()
                .Unwind("bar")
                .Match(BsonDocument.Parse("{ 'bar.barcode': '"+ barCode + "'}"))
                .Group(BsonDocument.Parse("{ '_id':'$fooCode' }"))

answered Mar 25, 2021 at 16:29

BSSchwarzkopf

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1 Comment

Rick DeBay Over a year ago

Thank you for your service, General. Is there a form where I could remove the strings from the query? Such that the Match's Parse is m=>bar.barCode==barcode. I ask because I've been in similar situation where the fully fluent query compiles, and exceptions at runtime.

Eagleclaw · Accepted Answer · 2021-03-24 20:22:43Z

1

You need to use aggregate in MongoDB. You can split the array elements with unwind, filter with match, select the keys that you want with project and group with common column like id or something.

MongoDB Aggregation docs: https://docs.mongodb.com/manual/aggregation/

answered Mar 24, 2021 at 20:22

Eagleclaw

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1 Comment

Rick DeBay Over a year ago

And how would that be implemented using the C# driver and account for a dynamic filter? That's where I'm stuck.

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