Solr: edismax boost not working

[ruby]

I'm playing with following query but can't get the *object_desc* field
boosted correctly. Maybe someone can tell me what I'm missing here:

&wt=json&&defType=edismax&indent=on
&q=object_name_NGRAM:video OR object_desc_NGRAM:video
&qf=object_desc_NGRAM^10 object_name^2

field definition:
<fieldType name="text_ngram" class="solr.TextField"
positionIncrementGap="100">
        <analyzer type="index">
                <tokenizer class="solr.StandardTokenizerFactory"/>
                <tokenizer class="solr.LowerCaseTokenizerFactory"/>
                <filter class="solr.EdgeNGramFilterFactory" minGramSize="1"
maxGramSize="50"/>
        </analyzer>
        <analyzer type="query">
                <tokenizer class="solr.StandardTokenizerFactory"/>
                <tokenizer class="solr.LowerCaseTokenizerFactory"/>
        </analyzer>
</fieldType>

<field indexed="true" multiValued="true"
         name="object_name_NGRAM"
         required="false"
         stored="false"
         type="text_ngram"/>
         
<field indexed="true" multiValued="true"
         name="object_desc_NGRAM"
         required="false"
         stored="false"
         type="text_ngram"/>


example documents:
<doc>
        <field name="id">1</field>
    <field name="object_name_NGRAM">video gaming</field>
    <field name="object_desc_NGRAM">economic</field>
</doc>


<doc>
        <field name="id">2</field>
    <field name="object_name_NGRAM">economic</field>
    <field name="object_desc_NGRAM">video</field>
</doc>

<doc>
        <field name="id">2</field>
    <field name="object_name_NGRAM">video gaming is an economic
business</field>
    <field name="object_desc_NGRAM">gaming</field>
</doc>


I want to boost to show the document having id=2. but my query is no
working. 



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