XSD 1.1: Use of attribute only when another attribute is present

I am supposed to write a xsd for a simple and small restaurant simulator. For this simulator I have a number of elements that are all considered to be food. Some of them need no further attributes since they are ready to go. However some of these food-elements need preparation and some of the elements that need preparation also should only be allowed to be prepared if something else is present. For this, they should be given the attribute "condition" (see example):

<food>
   <name>tea</name>
   <preparation>Ready to Go</preparation>
</food>

<food>
   <name>sandwich</name>
   <preparation time="2">Toaster</preparation>
</food>

<food>
   <name>noodles</name>
   <preparation time="5">Cooking-Pot</preparation>
</food>

<food>
   <name>lasagne</name>
   <preparation time="10" condition="noodles">Toaster</preparation>
</food>

I need to define it that the attribute condition can only be used, when the attribute time is present. So a element like the following should be invalid:

<food>
   <name>toast</name>
   <preparation condition="noodles">Toaster</preparation>
</food>

Furthermore, the attribute condition should only be able to have a value from the element "name" or "device" (and I don't know, if I solved that one since I cannot solve the first demand).

So far I tried defining both demands with an assert. But sadly it is not working at all. Either the attribute condition can be used any time and given any random value so it is ignoring all restrictions. Or the compiler (I'm working with Oxygen XML-Editor) always throws an error (Assertion evaluation for element 'food' ond schema type '#AnonType_foodmenues' did not succeed). Where did I make the mistake?

XSD:
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:vc="http://www.w3.org/2007/XMLSchema-versioning" elementFormDefault="qualified" vc:minVersion="1.1"> 
    <xs:element name="menues">
        <xs:complexType>
            <xs:sequence>
                <xs:element name="food" maxOccurs="unbounded" minOccurs="0">
                    <xs:complexType>
                        <xs:sequence>
                            <xs:element name="name">
                                <xs:simpleType>
                                    <xs:restriction base="xs:string">
                                        <xs:enumeration value="tea"/>
                                        <xs:enumeration value="sandwich"/>
                                        <xs:enumeration value="noodles"/>
                                        <xs:enumeration value="lasagne"/>
                                        <xs:enumeration value="pizza"/>
                                        <xs:enumeration value="juice"/>
                                        <xs:enumeration value="salat"/>
                                    </xs:restriction>
                                </xs:simpleType>
                            </xs:element>
                            <xs:element name="preparation">
                                <xs:complexType>
                                    <xs:simpleContent>
                                        <xs:extension base="device">
                                            <xs:attribute name="time">
                                                <xs:simpleType>
                                                    <xs:restriction base="xs:integer"/>
                                                </xs:simpleType>
                                            </xs:attribute>
                                            <xs:attribute name="condition" type="xs:string"/>
                                        </xs:extension>
                                    </xs:simpleContent>
                                </xs:complexType>
                            </xs:element>
                        </xs:sequence>
                        <xs:assert test="(preparation/@condition = (name, device)) or not (preparation/@condition)"/>
                        <xs:assert test="if (@condition) then (@time) else not (@condition)"/>
                    </xs:complexType>
                </xs:element>
            </xs:sequence>
        </xs:complexType>
    </xs:element>
    <xs:simpleType name="device">
        <xs:restriction base="xs:string">
            <xs:enumeration value="Ready to Go"/>
            <xs:enumeration value="Toaster"/>
            <xs:enumeration value="Cooking-Pot"/>
            <xs:enumeration value="Pan"/>
            <xs:enumeration value="Ofen"/>
        </xs:restriction>
    </xs:simpleType>
</xs:schema>

Since this is not working - how do I make it that the attribute condition can only be used when the attribute time is present?