如何在mongodb中查找最小值

sort

limit

min

> db.foo.insert({a: 1})
> db.foo.insert({a: 2})
> db.foo.insert({a: 3})
> db.foo.find().sort({a: 1}).limit(1) 
{ \"_id\" : ObjectId(\"4df8d4a5957c623adae2ab7e\"), \"a\" : 1 }

sort({a: 1})

a

  db.sales.insert([
    { \"_id\" : 1, \"item\" : \"abc\", \"price\" : 10, \"quantity\" : 2, \"date\" : ISODate(\"2014-01-01T08:00:00Z\") },
    { \"_id\" : 2, \"item\" : \"jkl\", \"price\" : 20, \"quantity\" : 1, \"date\" : ISODate(\"2014-02-03T09:00:00Z\") },
    { \"_id\" : 3, \"item\" : \"xyz\", \"price\" : 5, \"quantity\" : 5, \"date\" : ISODate(\"2014-02-03T09:05:00Z\") },
    { \"_id\" : 4, \"item\" : \"abc\", \"price\" : 10, \"quantity\" : 10, \"date\" : ISODate(\"2014-02-15T08:00:00Z\") },
    { \"_id\" : 5, \"item\" : \"xyz\", \"price\" : 5, \"quantity\" : 10, \"date\" : ISODate(\"2014-02-15T09:05:00Z\") }
  ])

  db.sales.aggregate(
   [
     {
       $group:
       {
         _id: {},
         minPrice: { $min: \"$price\" }
       }
     }
   ]
  );

{ \"_id\" : {  }, \"minPrice\" : 5 }

 db.sales.aggregate(
    [
      {
        $group:
        {
          _id: \"$item\",
          minQuantity: { $min: \"$quantity\" }
        }
      }
    ]
  )

{ \"_id\" : \"xyz\", \"minQuantity\" : 5 }
{ \"_id\" : \"jkl\", \"minQuantity\" : 1 }
{ \"_id\" : \"abc\", \"minQuantity\" : 2 }

[TestMethod]
public void Test()
{
  var _mongoServer = MongoServer.Create(\"mongodb://localhost:27020\");
  var database = _mongoServer.GetDatabase(\"StackoverflowExamples\");
  var col = database.GetCollection(\"items\");

  //Add test data
  col.Insert(new Item() { IntValue = 1, SomeOtherField = \"Test\" });
  col.Insert(new Item() { IntValue = 2 });
  col.Insert(new Item() { IntValue = 3 });
  col.Insert(new Item() { IntValue = 4 });

  var item = col.FindAs<Item>(Query.And())
  .SetSortOrder(SortBy.Ascending(\"IntValue\"))
  .SetLimit(1)
  .SetFields(\"IntValue\") //here i loading only field that i need
  .Single();
  var minValue = item.IntValue;

  //Check that we found min value of IntValue field
  Assert.AreEqual(1, minValue);
  //Check that other fields are null in the document
  Assert.IsNull(item.SomeOtherField);
  col.RemoveAll();
} 

Item

public class Item
{
   public Item()
   {
     Id = ObjectId.GenerateNewId();
   }

    [BsonId]
    public ObjectId Id { get; set; }
    public int IntValue { get; set; }
    public string SomeOtherField { get; set; }
}

public static class MongodbExtentions
{
    public static int FindMinValue(this MongoCollection collection, string fieldName)
    {
        var cursor = collection.FindAs<BsonDocument>(Query.And())
                     .SetSortOrder(SortBy.Ascending(fieldName))
                     .SetLimit(1)
                     .SetFields(fieldName);

        var totalItemsCount = cursor.Count();

        if (totalItemsCount == 0)
            throw new Exception(\"Collection is empty\");

        var item = cursor.Single();

        if (!item.Contains(fieldName))
            throw new Exception(String.Format(\"Field \'{0}\' can\'t be find within \'{1}\' collection\", fieldName, collection.Name));

        return item.GetValue(fieldName).AsInt32; // here we can also check for if it can be parsed
    }
}

[TestMethod]
public void Test()
{
  var _mongoServer = MongoServer.Create(\"mongodb://localhost:27020\");
  var database = _mongoServer.GetDatabase(\"StackoverflowExamples\");
  var col = database.GetCollection(\"items\");

  var minValue = col.FindMinValue(\"IntValue\");

  Assert.AreEqual(1, minValue);
  col.RemoveAll();
}