php - CodeIgniter 3.1.0 form validation issue

CodeIgniter 3.1.0 form validation issue.

I have an CodeIgniter application which use 2 databases:

1. Master database: "main_db".
2. Slave database: "client1", "client2"....

When user visit my website, I track the user url and get the client information. Then I connect to masterdatabase(main_db) and get the slave database information. After getting the correct slave database information, then I connect to slave database and the entire application use the slave database.

I have a table named "product_category". with a column name "category_name".

When I am trying to validate the form like

is_unique[product_category.category_name]
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When I run the script, the codeigniter use the master database whereas the using slave database is expected.

Table 'main_db.product_category' doesn't exist
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