sqlite - 在 Clojure 中通过 clojure.java.jdbc 使用外键约束

Question

我正在开发一个 wiki 程序并使用 SQLite 作为数据库。我想在 wiki 页面和描述这些页面的标签之间创建多对多关系。我使用 clojure.java.jdbc 来处理数据库操作。我想在页面到标签交叉引用表中强制执行外键约束。我查看了 SQLite 站点( https://www.sqlite.org/foreignkeys.html )上有关外键的信息，并相信这样的东西就是我想要的；

(def the-db-name "the.db")
(def the-db {:classname   "org.sqlite.JDBC"
             :subprotocol "sqlite"
             :subname     the-db-name})

(defn create-some-tables
  "Create some tables and a cross-reference table with foreign key constraints."
  []
  (try (jdbc/db-do-commands
         the-db false
         ["PRAGMA foreign_keys = ON;"
          (jdbc/create-table-ddl :pages
                                 [[:page_id :integer :primary :key]
                                  ;...
                                  [:page_content :text]])
          (jdbc/create-table-ddl :tags
                                 [[:tag_id :integer :primary :key]
                                  [:tag_name :text "NOT NULL"]])
          (jdbc/create-table-ddl :tags_x_pages
                                 [[:x_ref_id :integer :primary :key]
                                  [:tag_id :integer]
                                  [:page_id :integer]
                                  ["FOREIGN KEY(tag_id) REFERENCES tags(tag_id)"]
                                  ["FOREIGN KEY(page_id) REFERENCES pages(page_id)"]])])

       (catch Exception e (println e))))

但是尝试打开编译指示没有效果。

只是尝试打开编译指示并检查效果:

(println "Check before:" (jdbc/query the-db ["PRAGMA foreign_keys;"]))
; Transactions on or off makes no difference.
(println "Result of execute!:" (jdbc/execute! the-db
                                              ["PRAGMA foreign_keys = ON;"]))
(println "Check after:" (jdbc/query the-db ["PRAGMA foreign_keys;"]))

;=> Check before: ({:foreign_keys 0})
;=> Result of execute!: [0]
;=> Check after: ({:foreign_keys 0})

结果表明库(org.xerial/sqlite-jdbc“3.21.0.1”)已编译为支持外键，因为没有错误，但尝试设置编译指示没有效果。

我找到了this早在 2012 年，就出现在 clojure JDBC 的 JIRA 中。从那时起，所描述的更改已经实现，但代码仍然没有效果。

终于找到了指向 this post 的 Stackoverflow 问题的答案早在 2011 年。这让我能够拼凑出一些似乎确实设定了实用的东西。下面的代码取决于创建专门配置的连接。

(ns example
  (:require [clojure.java.jdbc :as jdbc])
  (:import (java.sql Connection DriverManager)
           (org.sqlite SQLiteConfig)))

(def the-db-name "the.db")
(def the-db {:classname   "org.sqlite.JDBC"
             :subprotocol "sqlite"
             :subname     the-db-name})

(defn ^Connection get-connection
  "Return a connection to a SQLite database that
  enforces foreign key constraints."
  [db]
  (Class/forName (:classname db))
  (let [config (SQLiteConfig.)]
    (.enforceForeignKeys config true)
    (let [connection (DriverManager/getConnection
                       (str "jdbc:sqlite:" (:subname db))
                       (.toProperties config))]
      connection)))

(defn exec-foreign-keys-pragma-statement
  [db]
  (let [con ^Connection (get-connection db)
        statement (.createStatement con)]
    (println "exec-foreign-keys-pragma-statement:"
             (.execute statement "PRAGMA foreign_keys;"))))

基于上述，我将上面的建表代码重写为:

(defn create-some-tables
  "Create some tables and a cross-reference table with foreign key constraints."
  []
  (when-let [conn (get-connection the-db)]
    (try
      (jdbc/with-db-connection
        [conn the-db]
        ; Creating the tables with the foreign key constraints works.
        (try (jdbc/db-do-commands
               the-db false
               [(jdbc/create-table-ddl :pages
                                       [[:page_id :integer :primary :key]
                                        [:page_content :text]])
                (jdbc/create-table-ddl :tags
                                       [[:tag_id :integer :primary :key]
                                        [:tag_name :text "NOT NULL"]])
                (jdbc/create-table-ddl :tags_x_pages
                                       [[:x_ref_id :integer :primary :key]
                                        [:tag_id :integer]
                                        [:page_id :integer]
                                        ["FOREIGN KEY(tag_id) REFERENCES tags(tag_id)"]
                                        ["FOREIGN KEY(page_id) REFERENCES pages(page_id)"]])])

             ; This still doesn't work.
             (println "After table creation:"
                      (jdbc/query the-db "PRAGMA foreign_keys;"))

             (catch Exception e (println e))))

      ; This returns the expected results.
      (when-let [statement (.createStatement conn)]
        (try
          (println "After creating some tables: PRAGMA foreign_keys =>"
                   (.execute statement "PRAGMA foreign_keys;"))
          (catch Exception e (println e))
          (finally (when statement
                     (.close statement)))))
      (catch Exception e (println e))
      (finally (when conn
                 (.close conn))))))

表已按预期创建。不过，一些 clojure.java.jdbc 函数似乎仍然无法按预期工作。 (请参阅 list 中间的 jdbc/query 调用。)让事情始终按预期工作似乎非常“手动”，必须依靠 java 互操作。似乎与数据库的每次交互都需要使用 get-connection 函数返回的专门配置的 Connection。

是否有更好的方法在 Clojure 中的 SQLite 中强制执行外键约束？

Answer 1

随着 next.jdbc 的出现你现在可以这样做:

(ns dev
  (:require [next.jdbc :as jdbc]
            [next.jdbc.sql :as sql]))

(with-open [conn (jdbc/get-connection {:dbtype "sqlite" :dbname "test.db"})]
  (println (sql/query conn ["PRAGMA foreign_keys"]))
  (jdbc/execute! conn ["PRAGMA foreign_keys = ON"])
  ; jdbc/execute whatever you like here...
  (println (sql/query conn ["PRAGMA foreign_keys"])))

此输出

[{:foreign_keys 0}]
[{:foreign_keys 1}]