Saving unique data in hasMany through model

Here's the associations that I have set up:

Partner hasMany PartnerInvolvement

Product hasMany PartnerInvolvement

PartnerInvolvementType hasMany PartnerInvolvement

PartnerInvolvement belongsTo Partner, Product, PartnerInvolvementType

The $hasMany relationship is defined the same way in all 3 of the Partner, Product, PartnerInvolvementType models:
class Product {

public $hasMany = array(

'PartnerInvolvement' => array(

'className' => 'PartnerInvolvement',

'foreignKey' => 'product_id',

'dependent' => true,

'unique' => 'keepExisting'

)

); 

}

When adding/editing a new Product, I want to be able to choose which Partners are involved with that Product, which PartnerInvolvementType it is, and create new PartnerInvolvement relationships from that data.

Here is the data that I am trying to save:

Array  (      [Product] => Array          (              [id] => 2981              [name] => Laurie Amerson Rental itr1229              [product_type_id] => 12              [default_price] => 0.00          )        [PartnerInvolvement] => Array          (              [0] => Array                  (                      [partner_id] => 1                      [partner_involvement_type_id] => 1                  )                [1] => Array                  (                      [partner_id] => 2                      [partner_involvement_type_id] => 1                  )            )    )    My app is creating a new PartnerInvolvement entry any time I try to save this data.  I am trying to restrain it so that a unique combination of product_id, partner_id, partner_involvement_type_id is only saved once.  Any suggestions?  Should I go back to using a HABTM instead of a hasMany through relationship?

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