title: PG:Undefined object error when using activeimport with constraint
body: <p>I am trying to do an upsert with ActiveImport. When running the import I am trying to pass it a named constraint in order to tell it when to update certain columns.</p>
  <pre><code> User.import(
        @import_values,
        on_duplicate_key_update: {
          constraint_name: :index_users_on_uid_and_tenant_id,
          columns: %i[title location_id email active]
        }
      )
  </code></pre>
  <p>The code above is my import statement which is correct according to activeimport documentation. However, when I run this, I get PG:UndefinedObject error which shows the following:</p>
  <pre><code>Caused by PG::UndefinedObject: ERROR:  constraint &quot;index_users_on_uid_and_tenant_id&quot; for table &quot;users&quot; does not exist
  </code></pre>
  <p>However, in my schema.rb, on my users table schema, the constraint clearly exists:</p>
  <pre><code> t.index [&quot;uid&quot;, &quot;tenant_id&quot;], name: &quot;index_users_on_uid_and_tenant_id&quot;, unique: true
  
  </code></pre>
  <p>and also, this returns true:</p>
  <pre><code>ActiveRecord::Base.connection.index_exists?(:users, [:uid, :tenant_id]) =&gt; true
  </code></pre>
  <p>Any help as to why this is happening would be very helpful!</p>