Working with related objects causing error

<?xml version="1.0" encoding="UTF-8"?>
<model package="storefinder" baseClass="xPDOObject" platform="mysql" defaultEngine="MyISAM" phpdoc-package="storefinder" phpdoc-subpackage="model" version="1.1">
  <object class="sfStore" table="sfinder_stores" extends="xPDOSimpleObject">
    <field key="name" dbtype="varchar" precision="100" phptype="string" null="false" default="" index="index" />
    <field key="address" dbtype="varchar" precision="255" phptype="string" null="false" default="" />
    <field key="city" dbtype="varchar" precision="255" phptype="string" null="false" default="" />
    <field key="state" dbtype="varchar" precision="255" phptype="string" null="false" default="" />
    <field key="zip" dbtype="varchar" precision="10" phptype="string" null="false" default="0" index="index" />
    <field key="country" dbtype="varchar" precision="20" phptype="string" null="false" default="" />
    <field key="phone" dbtype="varchar" precision="20" phptype="string" null="false" default="" />
    <field key="fax" dbtype="varchar" precision="20" phptype="string" null="false" default="" />
    <field key="active" dbtype="int" precision="1" attributes="unsigned" phptype="integer" null="false" default="0" />
    <alias key="postalcode" field="zip" />
    <index alias="name" name="name" primary="false" unique="false" type="BTREE">
        <column key="name" length="" collation="A" null="false" />
    </index>
    <index alias="zip" name="zip" primary="false" unique="false" type="BTREE">
        <column key="zip" length="" collation="A" null="false" />
    </index>
<composite alias="StoreOwners" class="sfStoreOwner" local="id" foreign="store" cardinality="many" owner="local" />
  </object>
  <object class="sfOwner" table="sfinder_owners" extends="xPDOSimpleObject">
    <field key="name" dbtype="varchar" precision="100" phptype="string" null="false" default="" index="index" />
    <field key="email" dbtype="varchar" precision="255" phptype="string" null="false" default="" />
    <index alias="name" name="name" primary="false" unique="false" type="BTREE">
        <column key="name" length="" collation="A" null="false" />
    </index>
<composite alias="StoreOwners" class="sfStoreOwner" local="id" foreign="owner" cardinality="many" owner="local" />
  </object>
  <object class="sfStoreOwner" table="sfinder_store_owners" extends="xPDOSimpleObject">
    <field key="store" dbtype="int" precision="10" attributes="unsigned" phptype="integer" null="false" default="0" index="index" />
    <field key="owner" dbtype="int" precision="10" attributes="unsigned" phptype="integer" null="false" default="0" index="index" />
    <index alias="store" name="store" primary="false" unique="false" type="BTREE">
        <column key="store" length="" collation="A" null="false" />
    </index>
    <index alias="owner" name="owner" primary="false" unique="false" type="BTREE">
        <column key="owner" length="" collation="A" null="false" />
    </index>
<aggregate alias="Store" class="sfStore" local="store" foreign="id" cardinality="one" owner="foreign" />
<aggregate alias="Owner" class="sfOwner" local="owner" foreign="id" cardinality="one" owner="foreign" />
  </object>
</model>

$modx->addPackage('storefinder', MODX_CORE_PATH . 'components/storefinder/model/');

$store = $modx->getObject('sfStore', 1);
$owners = $modx->getCollection('sfOwner');

$store->addMany($owners);
$store->save();

$category = $modx->getObject('modCategory',6);
$chunks = $modx->getCollection('modChunk');

$category->addMany($chunks);
$category->save();

<?xml version="1.0" encoding="UTF-8"?>
<model package="ofc" baseClass="xPDOObject" platform="mysql" defaultEngine="MyISAM" version="1.1">
<object class="classBlogposts" table="test_blogposts" extends="xPDOSimpleObject">

        <field key="content" dbtype="text" phptype="string" null="true" />

        <composite alias="aliasBlogpostsTags" class="classBlogpostsTags" local="id" foreign="blogpost" cardinality="many" owner="local" />
</object>
<object class="classTags" table="test_tags" extends="xPDOSimpleObject">

        <field key="name" dbtype="varchar" precision="255" phptype="string" null="true" />

        <composite alias="aliasTagsBlogposts" class="classBlogpostsTags" local="id" foreign="tag" cardinality="many" owner="local" />
</object>
<object class="classBlogpostsTags" table="test_blogposts_tags" extends="xPDOSimpleObject">
        <field key="blogpost" dbtype="int" precision="11" phptype="integer" null="true" />
        <field key="tag" dbtype="int" precision="11" phptype="integer" null="true" />
 
        <aggregate alias="Tags" class="classTags" local="tag" foreign="id" cardinality="one" owner="foreign" />
        <aggregate alias="Blogposts" class="classBlogposts" local="blogpost" foreign="id" cardinality="one" owner="foreign" />
</object>
</model>

$modx->addPackage('ofc', MODX_CORE_PATH . 'components/ofc/model/');

$blogpost = $modx->getObject('classBlogposts', 1);
$tags = $modx->getCollection('classTags');

$blogpost->addMany($tags);
$blogpost->save();

$blogpost = $modx->getObject('classBlogposts', 1);
$blogpostId = $blogPost->get('id');
$tags = $modx->getCollection('classTags');
 
foreach ($tags as $tag) {
    $tagId = $tag->get('id');
    $bpt = $modx->newObject('classBlogpostsTags');
    $bpt->set('blogpost', $blogpostId);
    $bpt->set('tag', $tagId);
    $bpt->save();
}

$blogpost->addMany($tags, 'aliasBlogpostsTags');