Pyspark：从涉及数组列的 Json 架构创建架构

Question

我在 json 文件中为 df 定义了我的架构，如下所示：

{
    "table1":{
        "fields":[
            {"metadata":{}, "name":"first_name", "type":"string", "nullable":false},
            {"metadata":{}, "name":"last_name", "type":"string", "nullable":false},
            {"metadata":{}, "name":"subjects", "type":"array","items":{"type":["string", "string"]}, "nullable":false},
            {"metadata":{}, "name":"marks", "type":"array","items":{"type":["integer", "integer"]}, "nullable":false},
            {"metadata":{}, "name":"dept", "type":"string", "nullable":false}       
        ]
    }

}

EG JSON 数据：

{
    "table1": [
        {
            "first_name":"john",
            "last_name":"doe",
            "subjects":["maths","science"],
            "marks":[90,67],
            "dept":"abc"        
        },
        {
            "first_name":"dan",
            "last_name":"steyn",
            "subjects":["maths","science"],
            "marks":[90,67],
            "dept":"abc"        
        },
        {
            "first_name":"rose",
            "last_name":"wayne",
            "subjects":["maths","science"],
            "marks":[90,67],
            "dept":"abc"            
        },
        {
            "first_name":"nat",
            "last_name":"lee",
            "subjects":["maths","science"],
            "marks":[90,67],
            "dept":"abc"        
        },
        {
            "first_name":"jim",
            "last_name":"lim",
            "subjects":["maths","science"],
            "marks":[90,67],
            "dept":"abc"        
        }       
    ]
}

我想从这个 json 文件创建等效的 spark 模式。下面是我的代码：（参考：从 json 模式表示创建 spark 数据帧模式）

with open(schemaFile) as s:
 schema = json.load(s)["table1"]
 source_schema = StructType.fromJson(schema)

如果我没有任何数组列，上面的代码工作正常。但是，如果我的架构中有数组列，则会引发以下错误。

“无法解析数据类型：数组”（“无法解析数据类型：%s”json_value）

Answer 1

在您的情况下，数组的表示存在问题。正确的语法是：

{ "metadata": {}, "name": "marks", "nullable": true, "type": {"containsNull": true, "elementType": "long", "type": "array" } }.

为了从 json 检索架构，您可以编写下一个 pyspark 代码段：

jsonData = """{
    "table1": [{
            "first_name": "john",
            "last_name": "doe",
            "subjects": ["maths", "science"],
            "marks": [90, 67],
            "dept": "abc"
        },
        {
            "first_name": "dan",
            "last_name": "steyn",
            "subjects": ["maths", "science"],
            "marks": [90, 67],
            "dept": "abc"
        },
        {
            "first_name": "rose",
            "last_name": "wayne",
            "subjects": ["maths", "science"],
            "marks": [90, 67],
            "dept": "abc"
        },
        {
            "first_name": "nat",
            "last_name": "lee",
            "subjects": ["maths", "science"],
            "marks": [90, 67],
            "dept": "abc"
        },
        {
            "first_name": "jim",
            "last_name": "lim",
            "subjects": ["maths", "science"],
            "marks": [90, 67],
            "dept": "abc"
        }
    ]
}"""

df = spark.read.json(sc.parallelize([jsonData]))

df.schema.json()

这应该输出：

{
    "fields": [{
        "metadata": {},
        "name": "table1",
        "nullable": true,
        "type": {
            "containsNull": true,
            "elementType": {
                "fields": [{
                    "metadata": {},
                    "name": "dept",
                    "nullable": true,
                    "type": "string"
                }, {
                    "metadata": {},
                    "name": "first_name",
                    "nullable": true,
                    "type": "string"
                }, {
                    "metadata": {},
                    "name": "last_name",
                    "nullable": true,
                    "type": "string"
                }, {
                    "metadata": {},
                    "name": "marks",
                    "nullable": true,
                    "type": {
                        "containsNull": true,
                        "elementType": "long",
                        "type": "array"
                    }
                }, {
                    "metadata": {},
                    "name": "subjects",
                    "nullable": true,
                    "type": {
                        "containsNull": true,
                        "elementType": "string",
                        "type": "array"
                    }
                }],
                "type": "struct"
            },
            "type": "array"
        }
    }],
    "type": "struct"
}

或者，您可以使用df.schema.simpleString()它返回一个相对简单的模式格式：

struct<table1:array<struct<dept:string,first_name:string,last_name:string,marks:array<bigint>,subjects:array<string>>>>

最后，您可以将上面的架构存储到一个文件中，然后使用以下命令加载它：

import json
new_schema = StructType.fromJson(json.loads(schema_json))

正如你所做的那样。请记住，您也可以为任何 json 数据动态地实现所描述的过程。