How to avoid showing fields in a customized form in OpenERP7?

I am using OpenERP7, and I have created a field in a form. This field is the next one:

'history': fields.function(_get_history, type='many2many',
                           obj="res.partner.link.category",
                           method=True, string='Categories'),

Then, I show it in the form. As the field is a many2many, it is shown as a tree, which I specify below.

<group string="Activity Summary">
    <field name="history" nolabel="1" attrs="{'readonly': 1}">
        <tree string="Categories">
            <field name="active_category" attrs="{'readonly': 1}"/>
            <field name="link_category_id" attrs="{'readonly': 1}"/>
            <field name="type" attrs="{'readonly': 1}"/>
            <field name="date" attrs="{'readonly': 1}"/>
            <field name="observations"/>
            <button name="open_history" type="object" string="View history" icon="terp-calendar"/>
        </tree>
    </field>
</group>

Everything is OK, but, if I click on one of the records, it is opened as a form in a popup, and it is showing some fields which I do not want. For example, the objects of "res.partner.link.category" have the attribute partner_id, which I do not want to be shown. So I did not write it inside the tree (which is working great), but I did the same for the form and this one is showing every attributes of "res.partner.link.category". Here the code after the modifications I did to show the form as I want:

<group string="Activity Summary">
    <field name="history" nolabel="1" attrs="{'readonly': 1}">
        <tree string="Categories">
            <field name="active_category" attrs="{'readonly': 1}"/>
            <field name="link_category_id" attrs="{'readonly': 1}"/>
            <field name="type" attrs="{'readonly': 1}"/>
            <field name="date" attrs="{'readonly': 1}"/>
            <field name="observations"/>
            <button name="open_history" type="object" string="View history" icon="terp-calendar"/>
        </tree>
        <form string="Categories" version="7.0">
            <sheet>
                <group col="4">
                    <field name="active_category"/>
                    <field name="link_category_id" options="{'no_open': True}"/>
                    <field name="type"/>
                    <field name="partner_id" attrs="{'invisible': True}"/>
                </group>
                <group col="4">
                    <field name="date"/>
                    <field name="observations"/>
                </group>
            </sheet>
        </form>
    </field>
</group>

What am I doing wrong? Is there any way to fix it?