我有以下问题：
>我得到了一个IQueryable< T>来自LinqToQueryString
>这个IQueryable< T>用于查询MongoDB
> IQueryable< T>用于返回分页数据集以及确定页面等的项目总数
> MongoDB在IQueryable< T> .Where().Count()上向Count添加一个group by.这会导致Count操作运行得非常慢.
可能的方法：
>获取表达式< Func< T,bool>>从最初的IQueryable< T>并将其应用于mongoCollection< T> .Count(过滤器).这绕过了这个问题.
我试图从IQueryable< T> .Expression中获取“Where”,然后将ExpressionType操作为可以在DynamicExpression.ParseLambda()中使用的格式.在大多数情况下,这工作正常,直到我使用DateTime表达式测试代码.
我附加了一个LINQPad脚本,该脚本使用本地MongoDB安装来填充数据,然后使用从ExpressionVisitor创建的新表达式计数.
我希望有一种更简单的方法可以在新的MongoDB FilterDefinitionBuilder< T> .Where(originalWhereExpression)中重用原始Expression中的“Where”.
脚本依赖是： 最佳答案 一旦理解了表达式树和可能的根级表达式方法名称,解决方案就相当简单.感谢@bolanki的帮助. 附件是更新的LINQPad脚本(测试集_doPrep = true)：
<Query Kind="Program">
<Reference><RuntimeDirectory>\System.Linq.dll</Reference>
<Reference><RuntimeDirectory>\System.Linq.Expressions.dll</Reference>
<Reference><RuntimeDirectory>\System.Linq.Queryable.dll</Reference>
<NuGetReference>Faker</NuGetReference>
<NuGetReference>mongocsharpdriver</NuGetReference>
<NuGetReference>MongoDB.Driver</NuGetReference>
<NuGetReference>NBuilder</NuGetReference>
<NuGetReference>Newtonsoft.Json</NuGetReference>
<NuGetReference>System.Linq.Dynamic</NuGetReference>
<Namespace>FizzWare.NBuilder</Namespace>
<Namespace>MongoDB.Bson</Namespace>
<Namespace>MongoDB.Bson.Serialization.Attributes</Namespace>
<Namespace>MongoDB.Driver</Namespace>
<Namespace>MongoDB.Driver.Builders</Namespace>
<Namespace>MongoDB.Driver.Linq</Namespace>
<Namespace>myAlias = System.Linq.Dynamic</Namespace>
<Namespace>Newtonsoft.Json</Namespace>
<Namespace>System.Linq</Namespace>
<Namespace>System.Linq.Expressions</Namespace>
<Namespace>System.Threading.Tasks</Namespace>
<Namespace>System.Threading.Tasks.Dataflow</Namespace>
</Query>
private string _mongoDBConnectionString = "mongodb://localhost";
private string _mongoDBDatabase = "LinqToQ";
private string _mongoDBCollection = "People";
private IMongoClient _mongoClient;
private IMongoDatabase _mongoDb;
private int _demoCount = 2000000;
private bool _doPrep = false;
void Main()
{
_connectToMongoDB();
// Should demo data be generated
if (_doPrep)
_prepMongo();
// Get the queryable to test with
var mongoDataQuery = _getIQueryable();
// Print the original expression
//mongoDataQuery.Expression.ToString().Dump("Original Expression");
// Evaluate the expression and try find the where expression
var whereFinder = new WhereFinder<Person>(mongoDataQuery.Expression);
// Get the MongoCollection to be Filtered and Count
var tempColl = _getPeopleCollection();
if (whereFinder.FoundWhere)
{
//whereFinder.TheWhereExpression.ToString().Dump("Calculated where expression");
var filter = new FilterDefinitionBuilder<Person>();
var stopwatch = new Stopwatch();
stopwatch.Start();
tempColl.Count(filter.Where(whereFinder.TheWhereExpression)).Dump("Dynamic where count");
var afterCalculatedWhere = stopwatch.Elapsed;
mongoDataQuery.Count().Dump("IQueryable<T> where count");
var afterIQuerableWhere = stopwatch.Elapsed;
stopwatch.Stop();
$"Calculated where:{afterCalculatedWhere:c}\nIQueryable where:{afterIQuerableWhere:c}".Dump("Where Durations");
}
else
tempColl.Count(FilterDefinition<Person>.Empty).Dump("No filter count");
"Done".Dump();
}
///////////////////////////////////////////////////////
// END SOLUTION
///////////////////////////////////////////////////////
private IMongoQueryable<Person> _getIQueryable()
{
var people = _getPeopleCollection();
//return people.AsQueryable().Where(p => p.DateOfBirth <= new DateTime(1974, 1, 1));
return people.AsQueryable().Where(p => p.LastName == "Anderson" && p.FirstName.Contains("f") && p.DateOfBirth >= new DateTime(1968, 1, 1) && p.DateOfBirth < new DateTime(1974, 1, 1));
//return people.AsQueryable().Where(p => p.LastName == "Anderson" && p.FirstName.Contains("f") && (p.DateOfBirth>=new DateTime(1968,1,1) && p.DateOfBirth<new DateTime(1974,1,1)));
//return people.AsQueryable().Where(p => p.LastName == "Anderson" && p.FirstName.Contains("f"));
//return people.AsQueryable().Where(p => p.FirstName.Contains("f"));
//return people.AsQueryable().Where(p => p.LastName == "Anderson");
}
public class Person
{
[BsonId]
public string Id { get; set; }
public string FirstName { get; set; }
public string LastName { get; set; }
public string Email { get; set; }
public DateTime DateOfBirth { get; set; }
}
public class WhereFinder<T> : MongoDB.Driver.Linq.ExpressionVisitor
{
private bool _processingWhere = false;
private bool _processingLambda = false;
public ParameterExpression _parameterExpression { get; set; }
public WhereFinder(Expression expression)
{
Visit(expression);
}
public Expression<Func<T, bool>> TheWhereExpression { get; set; }
public bool FoundWhere
{
get { return TheWhereExpression != null; }
}
protected override Expression VisitBinary(BinaryExpression node)
{
var result = base.VisitBinary(node);
if(_processingWhere)
TheWhereExpression = (Expression<Func<T, bool>>)Expression.Lambda(node, _parameterExpression);
return result;
}
protected override Expression VisitParameter(ParameterExpression node)
{
if (_processingWhere || _processingLambda || _parameterExpression==null)
_parameterExpression = node;
return base.VisitParameter(node);
}
protected override Expression VisitMethodCall(MethodCallExpression expression)
{
string methodName = expression.Method.Name;
if (TheWhereExpression==null && ( methodName == "Where" || methodName == "Contains"))
{
_processingWhere = true;
if (expression?.Arguments != null)
foreach (var arg in expression.Arguments)
Visit(arg);
_processingWhere = false;
}
return expression;
}
protected override Expression VisitLambda(LambdaExpression exp)
{
if (_parameterExpression == null)
_parameterExpression = exp.Parameters?.FirstOrDefault();
TheWhereExpression = (Expression<Func<T, bool>>)Expression.Lambda(exp.Body, _parameterExpression);
return exp;
}
}
///////////////////////////////////////////////////////
// END SOLUTION
///////////////////////////////////////////////////////
///////////////////////////////////////////////////////
// BEGIN DEMO DATA
///////////////////////////////////////////////////////
private void _prepMongo()
{
_mongoDb.DropCollection(_mongoDBCollection, CancellationToken.None);
var testData = _getDemoList(_demoCount);
var people = _getPeopleCollection();
people.Indexes.CreateOne(Builders<Person>.IndexKeys.Ascending(_ => _.FirstName));
people.Indexes.CreateOne(Builders<Person>.IndexKeys.Ascending(_ => _.LastName));
people.Indexes.CreateOne(Builders<Person>.IndexKeys.Ascending(_ => _.Email));
people.Indexes.CreateOne(Builders<Person>.IndexKeys.Ascending(_ => _.DateOfBirth));
$"Inserting ...{testData.Count}... demo records".Dump();
Extensions.ForEachOverTpl<Person>(testData, (person) =>
{
people.InsertOneAsync(person).Wait();
});
$"Inserted {testData.Count} demo records".Dump();
}
private IList<Person> _getDemoList(int demoCount)
{
var result = Builder<Person>.CreateListOfSize(demoCount)
.All()
.With(p => p.FirstName = Faker.NameFaker.FirstName())
.With(p => p.LastName = Faker.NameFaker.LastName())
.With(p => p.Email = Faker.InternetFaker.Email())
.With(p => p.DateOfBirth = Faker.DateTimeFaker.BirthDay(21, 50))
.Build();
return result;
}
private IMongoCollection<Person> _getPeopleCollection()
{
return _mongoDb.GetCollection<Person>(_mongoDBCollection);
}
private void _connectToMongoDB()
{
_mongoClient = new MongoClient(_mongoDBConnectionString);
_mongoDb = _mongoClient.GetDatabase(_mongoDBDatabase);
}
///////////////////////////////////////////////////////
// END DEMO DATA
///////////////////////////////////////////////////////
public static class Extensions
{
public static void ForEachOverTpl<T>(this IEnumerable<T> enumerable, Action<T> call)
{
var cancellationTokenSource = new CancellationTokenSource();
var actionBlock = new ActionBlock<T>(call, new ExecutionDataflowBlockOptions
{
TaskScheduler = TaskScheduler.Current,
MaxDegreeOfParallelism = Environment.ProcessorCount * 2,
CancellationToken = cancellationTokenSource.Token,
});
foreach (T item in enumerable)
{
if (cancellationTokenSource.IsCancellationRequested) return;
actionBlock.Post(item);
}
actionBlock.Complete();
actionBlock.Completion.Wait(cancellationTokenSource.Token);
}
}
更新： – 修复包含Take,OrderBy等的表达式.
public class WhereFinder<T> : MongoDB.Driver.Linq.ExpressionVisitor
{
private bool _processingWhere = false;
private bool _processingLambda = false;
public ParameterExpression _parameterExpression { get; set; }
public WhereFinder(Expression expression)
{
Visit(expression);
}
public Expression<Func<T, bool>> TheWhereExpression { get; set; }
public bool FoundWhere
{
get { return TheWhereExpression != null; }
}
protected override Expression Visit(Expression exp)
{
return base.Visit(exp);
}
protected override Expression VisitBinary(BinaryExpression node)
{
var result = base.VisitBinary(node);
if (_processingWhere)
{
TheWhereExpression = (Expression<Func<T, bool>>) Expression.Lambda(node, _parameterExpression);
}
return result;
}
protected override Expression VisitParameter(ParameterExpression node)
{
if (_processingWhere || _processingLambda || _parameterExpression == null)
_parameterExpression = node;
return base.VisitParameter(node);
}
protected override Expression VisitMethodCall(MethodCallExpression expression)
{
string methodName = expression.Method.Name;
if (methodName == "Where")
_processingWhere = true;
if (expression?.Arguments != null)
foreach (var arg in expression.Arguments)
Visit(arg);
_processingWhere = false;
return expression;
}
protected override Expression VisitLambda(LambdaExpression exp)
{
if (_processingWhere)
{
if (_parameterExpression == null)
_parameterExpression = exp.Parameters?.FirstOrDefault();
TheWhereExpression = (Expression<Func<T, bool>>)Expression.Lambda(exp.Body, _parameterExpression);
}
return exp;
}
}
