Custom ActiveRecord finder invoking named scopes?

I have a custom finder defined here: http://www.pastie.org/807187

Where search_by_date and search_by_text are named scopes, and eager is
an eager-loading named scope defined as:

named_scope :eager, :include => [{:container_inventory => {:container
=> [:size_type, :grade]}}, :company, :truck, :hauler]

The association is setup via a HMT (has_many :through):

class ContainerDepot << ActiveRecord::Base
  has_many :container_inventories
  has_many :container_gate_ins, :through => :container_inventories do
end

The problem is if the finder is invoked via association nesting from
ContainerDepot, it fails with an ActiveRecord::Statement::Invalid,
saying that the table has been specified more than once.

ContainerDepot.first.container_gate_ins.eager.search(text)
=> ActiveRecord::StatementInvalid: PGError: ERROR: table name
"container_inventories" specified more than once

I could correct it by copying the whole custom finder as an
association extension:

class ContainerDepot << ActiveRecord::Base
  ...
  has_many :container_gate_ins, :through => :container_inventories do
    def search(text)
      ... custom finder code from ContainerGateIn ...
    end
  end
  ...
end

it is not very DRY though and introduces a very unnecessary and
potentially problematic redundancy, as the custom finder will have to
be changed from time to time to accomodate additional search logic.

Any ideas on how this could be done better?