How to query array column from mysql in Rails 5.2?

Question

I am using gem rails~> 5.2 and gem mysql2 >= 0.3.13, < 0.5.

I have a model Lawer, which has an array column lawer_filed [sic].

# Model lawer.rb

serialize :lawer_field, Array

Then I created a Lawer, and I can get the lawer_field value as follows:

=> Lawer.first.lawer_field

=> ["2", "3", "5"] 

Now, I want to find one Lawer with a query using lawer_field. I tried:

@lawer = Lawer.where("lawer_field && ARRAY[?]", "2")

which raised an error like this:

ActiveRecord::StatementInvalid (Mysql2::Error: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '['2']) LIMIT 11' at line 1: SELECT  `lawers`.* FROM `lawers` WHERE (lawer_field && ARRAY['2']) LIMIT 11)

There is a mistake in my SQL syntax, but I don't how to fix it. Can anyone help?

Answer 1

MySQL, unlike PostgreSQL, does not support arrays in database. Therefore you needed to add this line:

serialize :lawer_field, Array

This means that you have a string field in your database, but whenever ActiveRecord is unpacking results returned by the database, it maps them directly to an instance of Ruby Array. What this means is that your only option to filter the results in the database is with any MySQL string comparison functions, LIKE, etc.

Your options are to either use LIKE or perform some other String functions (which will not perform well as you will be unable to use indices) or build another table, add a has_many association to it and use MySQL the way it was supposed to be used. You could also, of course, migrate to PostgreSQL, but that seems to be the most extreme option.

EDIT: you could also consider using MySQL`s JSON, which has been added recently. That depends on your version of MySQL though.

Answer 2

I didn't try this answer because I don't have any Rails project ready for testing, but I think the problem is in the syntax.

I think it should be something like this:

Lawer.where("lawer_field IN (?)", "2")