Mule 4：Dataweave 2.0：有什么方法可以在 groupBy 方法中使用类似于 SQL GroupBY 和 Have 的过滤条件？答案

【问题标题】：Mule 4 : Dataweave 2.0 : is there any way we can have a filter condition in groupBy method that works similar to SQL GroupBY and Having?Mule 4：Dataweave 2.0：有什么方法可以在 groupBy 方法中使用类似于 SQL GroupBY 和 Have 的过滤条件？
【发布时间】：2021-03-19 22:52:33
【问题描述】：

场景：给定如下输入 sampleArray ，我想将所有拥有特定老师的学生分组。

在 DataWeave 中，我们有一个方法 groupBy，它允许我们对指定字符串键的数组进行分组。但是在这里，由于 item.studentsMarks.subjectTeacher 返回一个数组，我收到了下面指定的错误。

谁能帮忙。提前致谢。

下面附上预期的输出。

示例输入：

var sampleArray = [
    {
        "studentName" : "ABC",  
        "studentsMarks" : [
            {
                "subject" : "maths",
                "marks" : "50",
                "subjectTeacher" : "teacher_1"
            },
            {
                "subject" : "science",
                "marks" : "30",
                "subjectTeacher" : "teacher_3"
            }
        ]
    },
    {
        "studentName" : "XYZ",  
        "studentsMarks" : [
            {
                "subject" : "maths",
                "marks" : "90",
                "subjectTeacher" : "teacher_1"

            },
            {
                "subject" : "arts",
                "marks" : "50", 
                "subjectTeacher" : "teacher_2"
            }
        ]
    }
]

代码尝试：

payload groupBy ((item, index) -> item.studentsMarks.subjectTeacher)

错误：

Cannot coerce Array to String

预期输出：

[
    
    "teacher_1" : [{
            "studentName" : "ABC",  
            "studentsMarks" : [
                {
                    "subject" : "maths",
                    "marks" : "50",
                    "subjectTeacher" : "teacher_1"
                },
                {
                    "subject" : "science",
                    "marks" : "30",
                    "subjectTeacher" : "teacher_3"
                }
            ]
        },    
        {
                "studentName" : "XYZ",  
                "studentsMarks" : [
                    {
                        "subject" : "maths",
                        "marks" : "90",
                        "subjectTeacher" : "teacher_1"
        
                    },
                    {
                        "subject" : "arts",
                        "marks" : "50", 
                        "subjectTeacher" : "teacher_2"
                    }
                ]
           }
    ],
    "teacher_2" : [
        
                {
                "studentName" : "XYZ",  
                "studentsMarks" : [
                    {
                        "subject" : "maths",
                        "marks" : "90",
                        "subjectTeacher" : "teacher_1"
        
                    },
                    {
                        "subject" : "arts",
                        "marks" : "50", 
                        "subjectTeacher" : "teacher_2"
                    }
                ]
           }
    ],
    "teacher_3" : [
        
{
            "studentName" : "ABC",  
            "studentsMarks" : [
                {
                    "subject" : "maths",
                    "marks" : "50",
                    "subjectTeacher" : "teacher_1"
                },
                {
                    "subject" : "science",
                    "marks" : "30",
                    "subjectTeacher" : "teacher_3"
                }
            ]
        }
    ]
        
]

【问题讨论】：

输出是什么样的？顺便说一句，问题出在item.studentsMarks.subjectTeacher 这个表达式上，因为item.studentsMarks 是一个数组——因此下一个.subjectTeacher 是一个与.*subjectTeacher 非常相似的选择器，它返回一个数组，因此你会得到你得到的错误。
@George 嗨，我附上了预期的输出。我理解了这个问题，但正在寻找一种使用 groupBy 实现解决方案的方法。如果您能找到合适的方法，请告诉我。非常感谢

标签： dataweave mule4

【解决方案1】：

这里有一个非常快速的方法来完成它。不过，它不仅仅需要groupBy：

%dw 2.0
output application/json

var sampleArray = [
    {
        "studentName" : "ABC",  
        "studentsMarks" : [
            {
                "subject" : "maths",
                "marks" : "50",
                "subjectTeacher" : "teacher_1"
            },
            {
                "subject" : "science",
                "marks" : "30",
                "subjectTeacher" : "teacher_3"
            }
        ]
    },
    {
        "studentName" : "XYZ",  
        "studentsMarks" : [
            {
                "subject" : "maths",
                "marks" : "90",
                "subjectTeacher" : "teacher_1"

            },
            {
                "subject" : "arts",
                "marks" : "50", 
                "subjectTeacher" : "teacher_2"
            }
        ]
    }
]
---
// Get list of all teachers
sampleArray..*subjectTeacher
// Change the collection to a set 
distinctBy $ 
// Iterate over every single teacher and create an object where
// the field is the teacher while the value is a collection
// containing all students where they attend a class for the teacher.
map {
    ($): (sampleArray dw::core::Arrays::partition (e) -> e..*subjectTeacher contains $).success
}

【讨论】：

你能解释一下你的方法吗？
用 cmets 注释代码。如果算法是问题，那么 cmets 会有所帮助。如果问题是对我使用的任何功能缺乏了解，请告诉我，我会添加文档链接。
cmets 将帮助任何访问者快速了解解决方案并将其用于他们的用例。我经历了这些方法并理解了它。感谢您的努力。