当字段类型在编译时未知时，如何使用tokio-postgres枚举列? [英] How to enumerate over columns with tokio-postgres when the field types are unknown at compile-time?

问题描述

我想要一个将SQL查询结果转换为JSON的通用函数.我想手动构建JSON字符串(或使用外部库).为此，我需要能够动态枚举一行中的列.

I would like a generic function that converts the result of a SQL query to JSON. I would like to build a JSON string manually (or use an external library). For that to happen, I need to be able to enumerate the columns in a row dynamically.

let rows = client
   .query("select * from ExampleTable;")
   .await?;

// This is how you read a string if you know the first column is a string type.
let thisValue: &str = rows[0].get(0);

使用Rust可以实现动态类型 ，但不能使用tokio-postgres库API.

Dynamic types are possible with Rust, but not with the tokio-postgres library API.

tokio-postgres的row.get函数旨在要求通用推断

The row.get function of tokio-postgres is designed to require generic inference according to the source code

如果没有正确的API，如何枚举行和列?

Without the right API, how can I enumerate rows and columns?

推荐答案

您需要枚举行和列，这样做可以在枚举时获取列引用，并从中获取postgresql类型.利用类型信息，可以有条件逻辑选择两个不同的子函数:i)获取强类型变量；并且，ii)转换为JSON值.

You need to enumerate the rows and columns, doing so you can get the column reference while enumerating, and from that get the postgresql-type. With the type information it's possible to have conditional logic to choose different sub-functions to both: i) get the strongly typed variable; and, ii) convert to a JSON value.

for (rowIndex, row) in rows.iter().enumerate() {
    for (colIndex, column) in row.columns().iter().enumerate() {
        let colType: string = col.type_().to_string();
        
        if colType == "int4" { //i32
            let value: i32 = row.get(colIndex);
            return value.to_string();
        }
        else if colType == "text" {
            let value: &str = row.get(colIndex);
            return value; //TODO: escape characters
        }
        //TODO: more type support
        else {
            //TODO: raise error
        }
    }
}

tokio-postgres代码维护者的奖金提示

理想情况下，tokio-postgres应该包含直接返回dyn any类型的API. row.rs内部已经使用数据库列类型信息来确认所提供的泛型类型有效.理想情况下，新API的使用将通过改进的FromSQL API直接直接使用内部列信息，但存在更简单的中间立场:-

Ideally, tokio-postgres would include a direct API that returns a dyn any type. The internals of row.rs already use the database column type information to confirm that the supplied generic type is valid. Ideally a new API uses would use the internal column information quite directly with improved FromSQL API, but a simpler middle-ground exists:-

row.rs中可能有一个额外的功能层，它使用此答案中使用的相同列类型条件逻辑来利用现有的get函数.如果像我这样的用户需要处理这种条件逻辑，则当tokio-postgresql处理新类型时，我也需要维护此代码，因此，应在库中包含此类逻辑，这样可以更好地实现此功能.保持.

It would be possible for an extra function layer in row.rs that uses the same column type conditional logic used in this answer to then leverage the existing get function. If a user such as myself needs to handle this kind of conditional logic, I also need to maintain this code when new types are handled by tokio-postgresql, therefore, this kind of logic should be included inside the library where such functionality can be better maintained.

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