处理DB查询结果集时抛出错误 [英] Error Thrown While Processing DB Query Resultset

问题描述

任何人都可以帮助我理解我的代码（见下文）为什么导致以下错误：

Can anyone help me understand why my code (see below) is resulting in the following error:

java.lang.UnsupportedOperationException：
nth不支持此类型：PersistentStructMap

Exception in thread "main" java.lang.UnsupportedOperationException: nth not supported on this type: PersistentStructMap

(defn search [query]
  (with-connection db 
    (with-query-results rs [query] 
      (doseq [[k v] rs] 
        (println v)))))

(search (nth *command-line-args* 0))

推荐答案

rs 是一个序列（列表），表示结果集中的所有记录。 rs 的每个元素都是一个表示单个记录的哈希表，地图中的键/值对表示该记录的字段名称和值。您尝试执行的操作相当于：

rs is a sequence (list), representing all the records in your resultset. Each element of rs is a hashmap representing a single record, with key/value pairs in the map representing field names and values for that record. You're trying to do the equivalent of this:

user> (let [rs [{:id 1 :val "foo"} {:id 2 :val "bar"}]]
        (doseq [[k v] rs]
          (println v)))
; Evaluation aborted.
; nth not supported on this type: PersistentArrayMap

这试图将每个映射解构为 [kv] ，大致相当于：

This is trying to destructure each map into [k v], doing the rough equivalent of this:

user> (let [k (nth {:id 1 :val "foo"} 0)
            v (nth {:id 1 :val "foo"} 1)])
; Evaluation aborted.
; nth not supported on this type: PersistentArrayMap

如果您尝试打印每个字段的值每个记录，您需要这样做：

If you're trying to print the value for every field in every record, you need to do this:

user> (let [rs [{:id 1 :val "foo"} {:id 2 :val "bar"}]]
        (doseq [record rs
                [k v] record]
          (println v)))
foo
1
bar
2

$ b b

对于结果集中的每个记录，对于该记录中的每个键/值，打印该值。

"For each record in the resultset, for each key/value in that record, print the value."

如果结果集只包含一个记录或者你只关心其中一个），并且你试图遍历该记录的字段，然后通过 doseq 只有第一：

If your resultset contains only a single record (or you only care about one of them) and you're trying to iterate over the fields of that single record, then pass doseq only the first:

user> (let [rs [{:id 1 :val "foo"}]]
        (doseq [[k v] (first rs)]
          (println v)))
foo
1

对于结果集的第一个记录中的每个键/值，请打印该值。

"For every key/value in the first record of the resultset, print the value."

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