MongoDB with C# - query with dynamically generated predicate

Question

I am having trouble querying a MongoDB database using a predicate which is dynamically generated.

The document structue i am trying to query on is:

{ 
    "_id" : 3121 , 
    "Active" : true , 
    "CategoryId" : 1 , 
    "Crci" : "IH" , 
    "CultureId" :  null  , 
    "DateUpdated" : { 
            "$date" : 1381916923120
    } , 
    "Description" : "National Careers Service: Actuary" , 
    "Keywords" : "" , 
    "MaxLevel" :  null  , 
    "MinLevel" :  null  , 
    "PhoneNumber" : "                    " , 
    "Priority" : 1 , 
    "Title" : "National Careers Service: Actuary" , 
    "WebUrl" : "https://nationalcareersservice.direct.gov.uk/advice/planning/jobprofiles/Pages/actuary.aspx" , 
    "CareerCultureExternalResources" : [ 
        { 
                "CareerId" : 5 , 
                "CultureId" : 1 , 
                "DisplayOrder" : 1 , 
                "ExternalResourceId" : 3121 , 
                "Vgs" :  null 
        }
    ] , 
    "SubjectExternalResources" : [ ] , 
    "LifestyleCategories" :  null
}

And I am trying to query using at predicate which is generated a runtime E.g.:

Expression<Func<ExternalResourceView, bool>> predicate = predicateBuilder.True<ExternalResourceView>();
if (request.CultureId != null && request.CareerId != null) {
    predicate = predicate.And(er => er.CareerCultureExternalResources.Any(ccer => ccer.CareerId == request.CareerId.Value && ccer.CultureId == request.CultureId.Value));
} 
else if (request.SubjectAreaId != null && request.SubjectLevel != null && request.CultureId != null)
{
    predicate = predicate.And(er => er.SubjectExternalResources.Any(ser => ser.SubjectAreaId == request.CareerId && ser.CultureId == request.CultureId));
}
IEnumerable<ExternalResourceView> matchingExternalResources = _externalResourceLibrary.AsQueryable().Where(predicate)

When I try and execute the query I get the error: "Unsupported where clause: <InvocationExpression>."

I am unable to find if the Linq to Mongo is able to generate a query from such an expression or not (I think it should).

So am I just trying to do something that the C# Mongo driver doesn't support or am I not generating the predicate right? I have seen from another question that what I'm trying to do should be possible (in theory at least).

I am using the official 10gen driver ver 1.8.3

Answer 1

Not sure it's a solution for mongodb, but could you try to use AsExpandable() coming from linqkit (made by the same guy as PredicateBuilder).

LinqKit (and more info about AsExpandable() and relation with PredicateBuilder) can be found here

Answer 2

Use the MongoDB Query builder instead of translating the predicate twice:

using MongoDB.Driver.Builders;
List<IMongoQuery> queries = new List<IMongoQuery>();
if (request.CultureId != null && request.CareerId != null) {
  queries.Add(
    Query<Person>.ElemMatch<IDependent>(p => p.Dependents,
      q => Query.And(
        q.EQ(x => x.CareerId, request.CareerId),
        q.EQ(x => x.CultureId, request.CultureId))));
}
// ... etc
// Final Query: 
var query = Query.And(queries);

I changed the query to use an $elemMatch, because I have the feeling you want to match both careerId and cultureId on the same array element. If that assumption is wrong, adjust the query accordingly.