Recently, I added full WordPress Multisite compatibility to my Email Log WordPress plugin and during the process, I learned a lot about how to create and handle tables in WordPress Multisite plugins. I thought of documenting it here so that it would be useful for other plugin authors.
Creating tables in Single site installations
Before we get into WordPress Multisite, first let’s see how we can create a table in a single site WordPress installation. You have to hook into the register_activation_hook action, which will be called every time you activate your plugin and then check if your table exists or not. If it doesn’t exist, then you can create your table.
The following code shows how you can do it.
The above code will work even in WordPress Multisite installations, if your plugin will be activated individually for each blog. But if your plugin is networked activated, then the above code will create the table only for the primary blog.
Creating tables for all blogs in a WordPress Multisite installation
Now that we know how to create the table for a single blog, let’s see how we can create the table for all the blogs in a WordPress Multisite installation. Even in this case, we have to hook into the same register_activation_hook action, but should loop through all the blogs in the network.
The following code shows how you can do it.
Creating table when a new blog is created
The above code will create the table only for the blogs that were created before the plugin got network activated. We should also make sure that we create the table for every new that gets created. In order to do that we can hook into the wpmu_new_blog action.
The following code shows how you can do it.
Deleting the table when a blog is deleted
Now that we are creating the table for every new blog, it is our job to make sure that the table is deleted when the blog is deleted. In order to do that, we can hook into the wpmu_drop_tables filter.
The following code shows how you can do it.
Querying the correct table
When we are querying the table, we should always use $wpdb->prefix . $table_name. If we do that, then WordPress will automatically query the correct table based on the current blog. We don’t have to manually find out the blog id and add it to the table name.
Now your plugin should be completely compatible with WordPress Multisite 🙂
Alternate approach
If you look into the above code closely, you will notice that we are creating one table for each blog in the network. For most WordPress Multisite installations, this shouldn’t be a problem. But some WordPress Multisite installation may have thousands and even hundreds of thousands of blogs. In those cases, we might end up creating huge amount tables which might become a bottleneck. Also my plugin needed just one table. Some plugins might need more than one table, which might also increase the number of tables that gets created.
One alternate approach is to create just one table for all blogs and then separate out data for each blog using a blog_id column. While querying the table, we can filter out based the blog_id column.
If I had started my plugin from scratch, I would have done that instead of creating separate tables.
Update: As Damian pointed out below in the comments, if you are using this approach, then you used use $wpdb->base_prefix to get the main prefix and not the individual sites prefix.
Removing the tables when the plugin is deleted
The other thing to keep in mind is that we should delete all the tables when the plugin is deactivated and deleted. I will write a separate article explaining how we should do that.
With WordPress Multisite becoming more popular these days, I hope this information was useful to you. Do let me know if you have any question or comments. Also you can checkout the entire code of my Email Log plugin in github.