如何用httr::POST批量获取REDCap调查链接？

问题描述 #

作为R语言新手，使用httr::POST调用REDCap API获取单个调查链接的代码可正常运行，但尝试通过循环批量获取多个record对应的链接时出现错误：

Error in vapply(elements, encode, character(1)) : values must be length 1, but FUN(X[[6]]) result is length 2

错误源于POST请求的参数编码，调用栈如下：

6. vapply(elements, encode, character(1))
5. compose_query(body)
4. body_raw(compose_query(body), "application/x-www-form-urlencoded")
3. body_config(body, match.arg(encode))
2. request_build("POST", hu$url, body_config(body, match.arg(encode)), 
as.request(config), ...)
1. httr::POST(url, body = formData, encode = "form")

原单次请求代码：

token <- "FC9439DCA5A64714XXXXXXXXXXXXX"
url <- "https://redcap.fmrp.usp.br/api/"
formData <- list("token"=token,
    content='surveyLink',
    format='xml',
    instrument= "qualidade_de_vida_cervical",
    event= "baseline_arm_1",
    record= "1",
    returnFormat='xml'
)
response <- httr::POST(url, body = formData, encode = "form")
result <- httr::content(response)
result <- minimal_html(result) %>% html_elements("body") %>% html_text2()
return(result)

尝试的批量代码（报错）：

my_list <- c("949_1","949_2")
    
for (i in my_list){
    token <- "FC9439DCA5A64XXXXXXXXXXXXXXXX"
    url <- "https://redcap.fmrp.usp.br/api/"
    formData <- list("token"=token,
        content='surveyLink',
        format='xml',
        instrument= "qualidade_de_vida_cervical",
        event= "baseline_arm_1",
        record= my_list,
        returnFormat='xml'
    )
    response <- httr::POST(url, body = formData, encode = "form")
    result <- httr::content(response)
    result <- minimal_html(result) %>% html_elements("body") %>% html_text2()
    return(result) 
}

错误原因 #

你在循环里犯了两个关键错误：

• record参数赋值错误：循环中你把整个my_list传给了record，而非当前迭代的元素i，导致record是长度为2的向量。REDCap的surveyLink接口单次请求仅接受单个record值，同时httr的encode="form"要求每个参数都是长度1的元素，这直接触发了编码错误。
• 循环内过早使用return()：return()会直接终止循环和函数运行，第一次迭代就会退出，根本不会处理第二个record。

解决方案 #

可以通过修正循环或使用迭代函数两种方式实现批量获取，具体如下：

方法1：修正for循环 #

library(httr)
library(rvest)

token <- "FC9439DCA5A64XXXXXXXXXXXXXXXX"
url <- "https://redcap.fmrp.usp.br/api/"
my_list <- c("949_1","949_2")
survey_links <- character(length(my_list))  # 预先创建存储结果的向量

for (i in seq_along(my_list)) {
    current_record <- my_list[i]
    formData <- list(
        "token" = token,
        content = 'surveyLink',
        format = 'xml',
        instrument = "qualidade_de_vida_cervical",
        event = "baseline_arm_1",
        record = current_record,
        returnFormat = 'xml'
    )
    response <- httr::POST(url, body = formData, encode = "form")
    result <- httr::content(response)
    survey_links[i] <- minimal_html(result) %>% html_elements("body") %>% html_text2()
}

# 查看结果
survey_links

方法2：使用purrr迭代函数 #

如果习惯函数式编程，用purrr::map()更简洁：

library(httr)
library(rvest)
library(purrr)

token <- "FC9439DCA5A64XXXXXXXXXXXXXXXX"
url <- "https://redcap.fmrp.usp.br/api/"
my_list <- c("949_1","949_2")

get_survey_link <- function(record_id) {
    formData <- list(
        "token" = token,
        content = 'surveyLink',
        format = 'xml',
        instrument = "qualidade_de_vida_cervical",
        event = "baseline_arm_1",
        record = record_id,
        returnFormat = 'xml'
    )
    response <- httr::POST(url, body = formData, encode = "form")
    result <- httr::content(response)
    minimal_html(result) %>% html_elements("body") %>% html_text2()
}

# 批量获取链接
survey_links <- map_chr(my_list, get_survey_link)

# 查看结果
survey_links

注意事项 #

• 确保REDCap项目权限允许批量获取调查链接，部分项目可能限制接口调用频率，若遇到限流问题，可在循环中加入Sys.sleep(1)之类的延迟。
• 始终保护token的安全性，不要将明文token提交到代码仓库或公开场合。

内容的提问来源于stack exchange，提问作者Gabriel Pokorny