mongodb c# select specific field

Question

Need some help creating a generic method for selecting fields by their name.

something like this:

T GetDocField<T>(string doc_Id, string fieldName)

The best I got is using projection which gives me the doc with only the wanted field seted:

 public T GetDocField<T>(string Doc_Id, string fieldName)
 {
    var value = DocCollection.Find(d => d.Id == Doc_Id)
               .Project<T>(Builders<Doc>.Projection
               .Include(new StringFieldDefinition<Doc>
               (fieldName))).FirstOrDefaultAsync().Result;

note: I'm using the new c# driver (2.0)

Thanks!!

If it's generic then why select by name and not by expression? — i3arnon
– i3arnon, Commented Jun 30, 2015 at 16:55
I have posted a new question about sub field selection. for the googlers out there: stackoverflow.com/questions/31161104/… — user4781762
– user4781762, Commented Jul 1, 2015 at 13:10

rnofenko · Accepted Answer · 2015-06-30 20:54:21Z

9

You can do next:

public async Task<TValue> GetFieldValue<TEntity, TValue>(string id, Expression<Func<TEntity, TValue>> fieldExpression) where TEntity : IEntity
{
    var propertyValue = await collection
        .Find(d => d.Id == id)
        .Project(new ProjectionDefinitionBuilder<TEntity>().Expression(fieldExpression))
        .FirstOrDefaultAsync();

    return propertyValue;
}

and call it

var value = await GetFieldValue<Item, string>("111", x => x.Name);

answered Jun 30, 2015 at 20:54

rnofenko

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3 Comments

user4781762 Over a year ago

This works perfect, but i wanted to go a little forward with this solution. I added an edit for the question , thanks again !

chridam Over a year ago

@AviPinhas It's considered bad practice here on SO to append a new issue to an existing question. Please edit out that second issue and post it in a new question. Again, would be great for you and the SO community to accept the answer above if you found it useful but there is no obligation in doing so.

user4781762 Over a year ago

I have posted a new question for sub field selection. for googlers who are interested: stackoverflow.com/questions/31161104/…

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