Extract list of selected contractors by matching serialized array in Rails
If your database supports array types natively I would use that  feature, I know that Postgres does. I would also call the field contractor_ids or even selected_contractor_ids then use a method called contractors or selected_contractors to return the actual objects.

I would change this:

@contractors = Contractor.where({ verified: true, validated: true })
                         .where('membership_type != ?', Contractor.membership_types["no_membership"])


to a scope or scopes on the Contractor class. You can also code the second where clause as where.not(membership_type: Contractor.membership_types["no_membership"])

And I would change this:

@selected_contractors = @company.contractors.map { |c| Contractor.find(c.to_i) }


to be a method on the Company class. I would also write it as:

@selected_contractors = Contractor.find(@company.contractors) 


this would issue a single query instead of n queries, additionally the to_i is not usually necessary.