How to query field equals empty array?

Question

I have an array field in my collection which have value with properties or empty value as "[]"

For fetching the documents having empty "[]" value in mongodb i use

db.getCollection('collection').find({"ArrayFieldName":{$eq:[]}})

This gives me result as expected. But when i try to form this query in the C# using mongodb driver i couldnt get the expected Result.

I tried, filterBuilder.Eq("ArraryFieldName", "[]") and filterBuilder.Eq("ArraryFieldName", new ArraryClassName(){})

Need help with C# filter builder to specify $eq:[] for arrary field.

ProgrammingLlama · Accepted Answer · 2021-03-10 06:21:38Z

4

An instance of ArraryClassName clearly won't work because it's not an array instance - it's a single object. Likewise "[]" won't work because it's a string.

You can check directly translate your CLI query to this filter:

Builders<BsonDocument>.Filter.Eq<BsonArray>("ArraryFieldName", new BsonArray())

Though if you simply want to check that the existing array is empty, you can use this filter instead:

Builders<BsonDocument>.Filter.Size("ArraryFieldName", 0)

P.S. I would strongly suggest using C# data models as it makes everything much easier to work with.

Also, you have called your field ArraryFieldName (notice the extra r at the end of Array). If you don't have existing data with this misspelled property name, you might want to correct the spelling.

answered Mar 10, 2021 at 6:21

ProgrammingLlama

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2 Comments

useruser00 Over a year ago

This worked Filter.Eq<BsonArray>("ArraryFieldName", new BsonArray())... I tried with Model class previously, but it not giving required value "[]" instead it coming with property names with default values.... BsonArray provided required output

ProgrammingLlama Over a year ago

You can still use string property names if needed (it's not obvious but it works), but with a model you can use something like model => model.ArrayFieldName, if that array field exists in your model object.

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