单元测试读取数据库时抛出SingleValueContainer错误求助

问题场景 #

单元测试中已成功向数据库写入AddressTypeTwo记录，但调用get方法读取时，抛出「decoding a single value container is not supported」错误。相关代码如下：

Codable结构体定义 #

struct AddressTypeTwo: Codable {
    // MARK: CRUD
    var id: Int?
    var created: Date?
    var createdBy: String?
    var modified: Date?
    var modifiedBy: String?
    var deleted: Date?
    var deletedBy: String?
    
    var name: String?
    var companyId: Int?
    
    // MARK: CodingKeys
    enum CodingKeys: String, CodingKey {
        case id, created, createdBy, modified, modifiedBy
        case deleted, deletedBy
        case name
        case companyId = "company_id"
    }
}

插入函数（运行正常） #

func insert(toBeInserted obj: Codable) -> Int {
    if let newId = try? DbSupport.shared.activeDatabase?.run(table.insert(obj)) {
        return Int(newId)
    }
    return -1
}

触发错误的读取函数 #

func get<T: Codable>(_ includeDeleted: Bool = false) -> [T]? {
    var query = table.filter(tableDeleted == nil)
    if includeDeleted {
        query = table
    }
    
    guard let database = DbSupport.shared.activeDatabase else { return nil }
    
    let addresses: [T?]? = try? database.prepare(query).map { row in
        if let rowData: T = try? row.decode() {
            return rowData
        }
        return nil
    }
    
    return addresses?.compactMap { $0 }
}

问题原因 #

错误的核心是row.decode()调用未明确指定解码目标类型，导致数据库库的解码逻辑默认尝试将行数据解析为单个值类型（如Int、String），但我们需要解码的是复合结构的AddressTypeTwo，二者不匹配从而触发错误。

解决方案 #

修改读取函数中的解码逻辑，明确指定要解码的泛型类型T.self：

func get<T: Codable>(_ includeDeleted: Bool = false) -> [T]? {
    var query = table.filter(tableDeleted == nil)
    if includeDeleted {
        query = table
    }
    
    guard let database = DbSupport.shared.activeDatabase else { return nil }
    
    do {
        let addresses: [T] = try database.prepare(query).map { row in
            try row.decode(T.self)
        }
        return addresses
    } catch {
        print("读取解码失败：\(error)")
        return nil
    }
}

额外检查点 #

1. Date类型解码匹配：如果数据库中Date字段存储的是时间戳（而非ISO8601字符串），需要配置Decoder的dateDecodingStrategy，确保与存储格式一致。例如：

let decoder = JSONDecoder()
decoder.dateDecodingStrategy = .secondsSince1970
// 可根据实际存储格式调整为.millisecondsSince1970或自定义策略

2. 表字段与CodingKeys匹配：确认数据库表的所有字段（如company_id）都存在，且数据类型与结构体属性对应（例如companyId是Int类型，表中字段也应为整数类型）。

内容的提问来源于stack exchange，提问作者iPhone Guy