Update mongodb

Src/Asp.NetCore2/MongoDbTest/UnitTest/QuerySelect.cs

@@ -164,6 +164,13 @@ namespace MongoDbTest
                    name = it.Name.Substring(0,1),  
                }).ToList();
             if(list17.First().name2!="a2"||list17.First().name.Substring(0,1)!= list17.First().name) Cases.ThrowUnitError();
+
+            var list18 = db.Queryable<Student>() 
+             .Select(it => new
+             { 
+                 name = it.Name.Length,
+             }).ToList();
+            if(list18.Max(it=>it.name)!=2) Cases.ThrowUnitError(); 
         }
         [SqlSugar.SugarTable("UnitStudent1231sds3z1")]
         public class Student : MongoDbBase

Src/Asp.NetCore2/SqlSugar.MongoDbCore/ExpToSql/VisitorItems/FieldPathExtractor.cs

@@ -38,7 +38,15 @@ namespace SqlSugar.MongoDb
                 var value = ExpressionTool.GetMemberValue(oldMember.Member, oldExp);
                 return UtilMethods.MyCreate(value);
             }
-            else if (!string.IsNullOrEmpty(BinaryExpressionTranslator.GetSystemDateMemberName(expr)))
+            else if (IsLength(oldMember))
+            {
+                var memberExp = (expr as MemberExpression);
+                var method = new MongoDbMethod() { context = _context };
+                var model = new MethodCallExpressionModel() { Args = new List<MethodCallExpressionArgs>() };
+                model.Args.Add(new MethodCallExpressionArgs() { MemberValue = memberExp.Expression });
+                return BsonDocument.Parse(method.Length(model));
+            }
+            else if (IsDateProperty(expr))
             {
                 var memberExp = (expr as MemberExpression);
                 var method = new MongoDbMethod() { context = _context };
@@ -58,6 +66,26 @@ namespace SqlSugar.MongoDb
             return ExtractFieldPath(expr);
         }
 
+        private static bool IsDateProperty(Expression expr)
+        {
+            return !string.IsNullOrEmpty(BinaryExpressionTranslator.GetSystemDateMemberName(expr));
+        }
+
+        private static bool IsLength(MemberExpression oldMember)
+        {
+            if (oldMember.Member.Name != "Length")
+                return false;
+
+            var expressionType = oldMember.Expression?.Type;
+            if (expressionType == null)
+                return false;
+
+            return expressionType.IsArray
+                || expressionType == typeof(string)
+                || expressionType.FullName == "System.Span`1"
+                || expressionType.FullName == "System.ReadOnlySpan`1";
+        }
+
         private BsonValue ExtractFieldPath(Expression expr)
         {
             var parts = new Stack<string>();