Data change history with audit tables: Grouping changes

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有刺的猬
有刺的猬 2021-02-06 08:01

Lets say I want to store users and groups in a MySQL database. They have a relation n:m. To keep track of all changes each table has an audit table user_journal, group_journal a

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  • 2021-02-06 08:41

    Assuming you're rate of adding a batch of users to a group is less than once a second....

    I would suggest simply adding a column of type timestamp named something like added_timestamp to the user_group and user_group_journal. DO NOT MAKE THIS AN AUTO UPDATE TIMESTAMP OR DEFAULT IT TO CURRENT_TIMESTAMP, instead, in your code when you insert by batch into the user_group, calculate the current date and time, then manually set this for all the new user_group record.

    You may need to tweak your setup to add the field to be copied the rest of the new user_group record into the user_group_journal table.

    Then when you could create a query/view that groups on a group_id and the new added_timestamp column.

    If more fidelity is needed then 1 second you could use a string column and populate it with a string representation of a more granular time (which you'd need to generate however the libraries your language of use allows).

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  • 2021-02-06 08:58

    I played a bit around and now I found a very good solution:

    The Database setup

    # First of all I create the database and the basic table:
    
    DROP DATABASE `mytest`;
    CREATE DATABASE `mytest`;
    USE `mytest`;
    CREATE TABLE `test` (
        `id` INT PRIMARY KEY AUTO_INCREMENT,
        `something` VARCHAR(255) NOT NULL
    );
    
    # Then I add an audit table to the database:
    
    CREATE TABLE `audit_trail_test` (
        `_id` INT PRIMARY KEY AUTO_INCREMENT,
        `_revision_id` VARCHAR(255) NOT NULL,
        `id` INT NOT NULL,
        `something` VARCHAR(255) NOT NULL
    );
    
    # I added a field _revision_id to it. This is 
    # the ID that groups together all changes a
    # user made within a request of that web
    # application (written in PHP). So we need a
    # third table to store the time and the user
    # that made the changes of that revision:
    
    CREATE TABLE `audit_trail_revisions` (
        `id` INT PRIMARY KEY AUTO_INCREMENT,
        `user_id` INT NOT NULL,
        `time` DATETIME NOT NULL
    );
    
    # Now we need a procedure that creates a
    # record in the revisions table each time an
    # insert or update trigger will be called.
    
    DELIMITER $$
    
    CREATE PROCEDURE create_revision_record()
    BEGIN
        IF @revision_id IS NULL THEN
            INSERT INTO `audit_trail_revisions`
                (user_id, `time`)
                    VALUES
                (@user_id, @time);
            SET @revision_id = LAST_INSERT_ID();
        END IF;
    END;
    
    # It checks if a user defined variable
    # @revision_id is set and if not it creates
    # the row and stores the generated ID (auto
    # increment) into that variable.
    # 
    # Next I wrote the two triggers:
    
    CREATE TRIGGER `test_insert` AFTER INSERT ON `test` 
        FOR EACH ROW BEGIN
            CALL create_revision_record();
            INSERT INTO `audit_trail_test`
                (
                    id,
                    something,
                    _revision_id
                ) 
            VALUES
                (
                    NEW.id,
                    NEW.something,
                    @revision_id
                );
        END;
    $$
    
    CREATE TRIGGER `test_update` AFTER UPDATE ON `test` 
        FOR EACH ROW BEGIN
            CALL create_revision_record();
            INSERT INTO `audit_trail_test`
                (
                    id,
                    something,
                    _revision_id
                ) 
            VALUES
                (
                    NEW.id,
                    NEW.something,
                    @revision_id
                );
        END;
    $$
    

    The application code (PHP)

    $iUserId = 42;
    
    $Database = new \mysqli('localhost', 'root', 'root', 'mytest');
    
    if (!$Database->query('SET @user_id = ' . $iUserId . ', @time = NOW()'))
        die($Database->error);
    if (!$Database->query('INSERT INTO `test` VALUES (NULL, "foo")'))
        die($Database->error);
    if (!$Database->query('UPDATE `test` SET `something` = "bar"'))
        die($Database->error);
    
    // To simulate a second request we close the connection,
    // sleep 2 seconds and create a second connection.
    $Database->close();
    sleep(2);
    $Database = new \mysqli('localhost', 'root', 'root', 'mytest');
    
    if (!$Database->query('SET @user_id = ' . $iUserId . ', @time = NOW()'))
        die($Database->error);
    if (!$Database->query('UPDATE `test` SET `something` = "baz"'))
        die($Database->error);
    

    And … the result

    mysql> select * from test;
    +----+-----------+
    | id | something |
    +----+-----------+
    |  1 | baz       |
    +----+-----------+
    1 row in set (0.00 sec)
    
    mysql> select * from audit_trail_test;
    +-----+--------------+----+-----------+
    | _id | _revision_id | id | something |
    +-----+--------------+----+-----------+
    |   1 | 1            |  1 | foo       |
    |   2 | 1            |  1 | bar       |
    |   3 | 2            |  1 | baz       |
    +-----+--------------+----+-----------+
    3 rows in set (0.00 sec)
    
    mysql> select * from audit_trail_revisions;
    +----+---------+---------------------+
    | id | user_id | time                |
    +----+---------+---------------------+
    |  1 |      42 | 2013-02-03 17:13:20 |
    |  2 |      42 | 2013-02-03 17:13:22 |
    +----+---------+---------------------+
    2 rows in set (0.00 sec)
    

    Please let me know if there is a point I missed. I will have to add an action column to the audit tables to be able to record deletions.

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