3.0: Saving new entity with belongsToMany data

i've seen a couple similar threads about this, but not my exact problem.

i'm trying to follow the guide here: http://book.cakephp.org/3.0/en/orm/saving-data.html#converting-request-data-into-entities and coming up short. i'm not getting any errors (except a date field that constantly fails if it has a value, separate issue)...so i have no idea what's happening to the data i submit, or why it isn't saved to the DB.

My raw response data logged to Cake Debug is:

Array (
  [title] => '123 Main St'
  [street] => '123 Main St'
  [lot_no] => 1
  [lat] =>
  [lng] =>
  [city] => 'Hometown'
  [county] =>
  [state] => 'MI'
  [zip] => '49000'
  [area] => 900    
  [bedrooms] => 2    
  [bathrooms] => 1    
  [price] => 525    
  [ready_date] =>
  [listingsattrs] => Array (
    [4] => Array (
      [id] => 4
    )
    [1] => Array (
      [id] => 1
      [value] => 'Off-street'
    )
  )
  [park_id] => 1
  [house_condition_id] => 3
  [house_style_id] => 2
  [term_id] => 2
  [info_body] => '<p>Live here. It&#39;s great!</p>'
)

i have tables Listings, ItemAttrs, and ListingsAttrs. In ListingsTable, the relationship is defined as:

$this->addAssociations([
  'belongsToMany' => [
    'ItemAttrs' => [
      'targetForeignKey' => 'attr_id',
      'through'          => 'ListingsAttrs',
      'saveStrategy'     => 'replace',
  ]
])'

Some attributes are required to have a value when assigned to a Listing (or other parent object they can be assigned to with a similar relationship). When a requires-value attribute is selected for the (in this case) Listing, a text field is shown & marked as required. The value is stored in the ListingsAttrs record with the Listing ID and ItemAttr ID.

When i submit the form, the correct attribute data is there. But it gets lost when i generate the Entity:

$listing = $this->Listings->newEntity($this->request->data, [
  'associated' => ['ListingsAttrs']
]);

Log of $listing:

{
  "title": "123 Main St",
  "street": "123 Main St",
  "lot_no": "1",
  "lat": null,
  "lng": null,
  "city": "Hometown",
  "county": null,
  "state": "MI",
  "zip": "49000",
  "area": 900,
  "bedrooms": 2,
  "bathrooms": 1,
  "price": 525,
  "ready_date": null,
  "park_id": 1,
  "house_condition_id": 3,
  "house_style_id": 2,
  "term_id": 2,
  "info_body": "<p>Live here. It&#39;s great!<\/p>\r\n"
}

i've tried creating the Listings entity without 'associated' at all, as 'associated' => 'ListingsAttrs', and 'associated' => 'ItemAttrs'

i'm trying to understand this ORM, because it looks like it should be very automated & easy to use. (By comparison, has anyone here used ZF1's ORM? Yikes.) But i don't know what i'm doing wrong here. It seems like i'm following the guide, but i'm obviously mixing something up.

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