Querying objects

[Vil]

Hi,

Is it possible to make a query on a field that is located in a child object? 

Example:

class Person{

 string name;

 int age;

 Address address;

}

class Address{

  Long id;

  int num;

  String street;
  String city;

  String postcode;

}

I would like to find the People who have an  Address.num equal to 85?

What is the best approach to do that with CQEngine? 

Thanks

Vincent

I 

[Niall Gallagher]

Hi Vil,

Yes, you can write an attribute which does that.

The attribute will be of type <Person, Integer>. It will be supplied the Person object, and its implementation should return person.address.num.

Basically, think of an attribute as a function. It is supplied a Person object, and its task is to return a value associated with that person (the value of person.address.num in this case).

HTH,
Niall

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[Vil]

Hi Niall,

Thanks for your answer!

In the case of a 1-1 relationship between objects it's clear!

But what's happens for a relationship 1-*?

In my example could we consider a list of Address instead of an Address for the Person

class Person{

 string name;

 int age;

 List<Address> addresses;

}

class Address{

  Long id;

  int num;

  String street;
  String city;

  String postcode;

}

What is the approach to  find People who have an  Address.num equal to 85?

Regards

Vincent

[Niall Gallagher]

Hi Vincent,

Use a MultiValueAttribute in that case. It will work.

Thanks,

Niall

[Vil]

Hi Niall,

Ok for the MultiValueAttribute. 

But now, how i could use this attribute to retrieve Person with a num address equal to 85?? 

class Person{

 string name;

 int age;

 List<Address> addresses;

 static Attribute<Person, Address> ADDRESS = MultiValueAttribute<Person, Address>(){

       public List<Address> getValues(Person person) {

            return person.addresses;

        }

    };

}

class Address{

  Long id;

  int num;

  static Attribute<Address, Integer> NUM = SimpleValueAttribute<Address, Integer>(){

       public Integer getValues(Address address) {

            return address.num;

        }

    };

}

IndexedCollection persons = CQEngine.newInstance();

persons = fillWithPerson();

persons.retrieve(?????); // get persons with num address equal to 85

Regards

Vincent

[Niall Gallagher]

Hi Vincent,

You don't need an attribute of type <Address, Integer>.

You just need one attribute of type <Person, Integer>:

 static Attribute<Person, Integer> ADDRESS_NUMBER = MultiValueAttribute<Person, Integer>(){

       public List<Integer> getValues(Person person) {

            return <a list of the address numbers for this person>;

        }

    };

}

...

persons.retrieve(equal(Person.ADDRESS_NUMBER, 85)); // get persons with num address equal to 85

--

[Vil]

Ok! Thanks a lot Niall!

Vincent

[Vil]

If now i would like to make a query to retrieve Person who have an Address num equal to 85 in the city "NEW YORK"?

I could use :

persons.retrieve(and(equal(Person.ADDRESS_NUMBER, 85), equal(Person.ADDRESS_CITY, "NEW YORK");

But i think the results will not be the expected result. The query retrieve a list of Person who have a num address equal to 85 and an address in "NEW YORK" (but not necessarily tat num 85!).

Is there an issue?

Regards

Vincent

Le vendredi 11 avril 2014 13:22:37 UTC+2, Niall a écrit :

[Niall Gallagher]

Yes that query is like the following in SQL:

SELECT * FROM Person
WHERE EXISTS SELECT 1 FROM Addresses WHERE Address.personId = Person.personId AND Address.city = 'New York'

AND EXISTS SELECT 1 FROM Addresses WHERE Address.personId = Person.personId AND Address.num = 85;

You need something like:

SELECT * FROM Person
WHERE EXISTS SELECT 1 FROM Addresses WHERE Address.personId = Person.personId AND Address.city = 'New York' AND Address.num = 85;

So you need to create an association in your query. One option is to define an attribute of type <Person, Pair<String, Integer>>, where the pair is String address.city, Integer address.num.

Then you can query the collection by supplying the sought combination of city and num:
persons.retrieve(equal(Person.CITY_NUM, Pair.of("New York", 85));

Another option is to break out addresses into a separate IndexedCollection, and do a JOIN between the collections.

HTH,
Niall