c# - 在 C# MongoDB 2.0 中的多个字段上进行项目

Question

当字段以字符串数组的形式给出时，如何在新的 MongoDB C# 驱动程序中投影字段？我可以通过做找到在单个领域进行投影的方法

collection.find(filter).Project(Builders<Category>.Projection.Include(fieldName)

如何扩展它以获取字段数组？

Answer 1

还有扩展方法Include

var projection = Builders<Category>.Projection.Include(fieldList.First());
foreach (var field in fieldList.Skip(1))
{
    projection = projection.Include(field);
}
var result = await collection.Find(filter).Project(projection).ToListAsync();

Answer 2

比 mofenko 更好的方法，您不必包含第一列：

ProjectionDefinition<BsonDocument> project = null;

foreach (string columnName in columnsToInclude)
{
    if (project == null)
    {
        project = Builders<BsonDocument>.Projection.Include(columnName);
    }
    else
    {
        project = project.Include(columnName);
    }
}

这适用于松散类型的数据。如果您使用类替换BsonDocument为您的类

Answer 3

另一种方式，假设 fieldList 是一个可枚举的字符串：

var project = Builders<BsonDocument>.Projection.Combine(fieldList.Select(x => Builders<BsonDocument>.Projection.Include(x)).ToList());