Let's say I have model A, Model B and Model C
class ModelA(ModelB):
data = mongoengine.ReferenceField()
class ModelB(Document):
customer = mongoengine.ReferenceField(ModelC)
class ModelC(Document):
name = mongoengine.stringField()
I'm able to access this.
models.ModelA.objects(customer=customer)
Now I'm trying to filter where customer.name is equal to a name I pass in. Is this possible with mongoengine filters?
I tried using this method but it wont work
models.ModelA.objects(customer__name=name)
No this is not possible since joins dont exist in mongodb, you need do this in 2 step, first querying the customer based on the name, then query ModelA based on the outcome of the first query.
customer = ModelC.objects.get(name=your_name)
ModelA.objects(customer=customer)
Related
I have researched this for a few days but can't seem to find the right information.
Here is what I need, I have a Database, with multiple tables, I need to join a few tables together to make a sort of "search" API. I have to implement the ability to dynamically search fields (from various tables in the query), sortable, with pagination.
I have found that I cannot combine the #Query annotation with Specification API, and I looked into using the specification API to do the joins I needed but, the problem is the root must be one table/repository.
For example:
If I have a users table that has to join on addresses, phone_numbers, and preferences
the base repository will be UserResposiory and it will return the User entity model, but I need it to return a custom DTO
AccountUserDTO which contains fields from the User, Address, PhoneNumber, and Preference entities.
Would anyone know if this is possible at all??
I am at wits end here and I really want to build this the correct way.
Cheers!
You may do this way:
Build hql query as an string, depend on how the filter condition is requested, you can build the corresponding query, eg:
if (hasParam(searchName)) {
queryString = queryString + " myEntity.name = :queryName"
}
Query query = session.createQuery(queryString);
and the parameter providing
if (hasParam(searchName)) {
query.setParameter("queryName", searchName);
}
...
and execute it.
To create a customized object, the easiest way is treating the object as an array of field:
Query query = session.createQuery("select m.f1, m.f2, m.f3 from myTable m");
List managers = query.list();
Object[] manager = (Object[]) managers.get(0); //first row
System.out.println(manager[0]) //f1
System.out.println(manager[1]) //f2
System.out.println(manager[2]) //f3
There is also some other solution to select, such as
String query = "select new mypackage.myclass(m.f1, m.f2, m.f3) from myTable m";
-> And when execute the above query, it will return a list of object.
Or to be simpler, make your own view in db and map it to one entity.
I don't know whether I should be drawing parallels, but unfortunately, that's the only way I can express my issue.
In SQL, suppose we have two tables:
Employee with columns Employee ID, Employee Name, Dept. ID
Deptartment with columns Dept. ID, Dept Name
The Dept ID. in the Employee table is a foreign key with that in the Department table.
Now suppose I want to fetch the following columns:
Employee ID, Employee Name, Department Name
using a SQL such as:
SELECT A.EMPLOYEE_ID, A.EMPLOYEE_NAME, B.DEPT_NAME
FROM EMPLOYEE A, DEPARTMENT B
WHERE A.DEPT_ID = B.DEPT_ID
How would one do this using Core Data in Swift? I guess I'm getting confused by only seeing references to
NSFetchRequest(entityName: entityName)
where the entityName refers to a single entity (table in relational DB lingo)
I'd really appreciate any pointers or examples that can help me clear my doubts.
Thanks
It is certainly possible to create a fetch request that is equivalent to your SQL query. More complex queries can be difficult if not impossible to achieve with a single fetch request. But I recommend trying NOT to draw parallels between CoreData and SQL, at least until you have got to grips with how it works.
To take your example, in the CoreData view of the world, Employee would be an entity with a relationship to another entity, Department. A fetch request based on the Employee entity will return an array of Employee objects, and (assuming you create subclasses of NSManagedObject for each entity) you can access the attributes with simple dot notation:
let employeeName = myEmployeeObject.employeeName
But you can use the same notation to traverse relationships equally easily:
let departmentName = myEmployeeObject.department.departmentName
You don't need to worry about joins, etc; CoreData handles that for you.
Now, suppose you try to do it "the SQL way". You can construct a fetch request based on the Employee entity, but specify "properties to fetch" which likewise traverse the relationship:
let fetch = NSFetchRequest(entity:"Employee")
fetch.propertiesToFetch = ["employeeID", "employeeName", "department.departmentName"]
For this to work, you would need to specify the "resultType" for the fetch request to be DictionaryResultType:
fetch.resultType = .DictionaryResultType
The results from this query would be an array of dictionaries containing the relevant keys and values. But the link with the underlying objects is lost. If you subsequently want to access any details from the relevant Department (or even the Employee), you would have to run a new fetch to get the object itself.
I have a Customer table and another Orders table. Each Customer can have many orders (One to many relationship).
I want to get a Customer object and from it get how many orders he has (the actual order data is not relevant at this point). So as I see it I have 2 options:
create a view with another OrdersCount field - and that will be another object in my system.
in my app, when I need the count get the Customer.Orders.Count - but for my understanding that will cause an extra query to run and pull all the orders from the database to that collection.
Is there a correct way to do such thing?
Thanks
You do need a new type, but you don't need to recreate all relevant properties.
from c in context.Customers
// where ...
select new {
Customer = c,
OrderCount = c.Orders.Count()
}
Update code that looks for e.g. the Name property of an item in the result, to look for Customer.Name.
I have an entity that has a relationship with another entity. I am able to search on columns that are in the main entity, and include columns from the relationship entity. But I need to be able to filter the list (search) on columns that are not in the relationship entity.
for example
the Invoice Entity contains a customerId property, and is related to the Customer Entity which contains the customerName property
I need to be able to search / filter the grid by customerName.
I am new to entity framework, please help.
thanks
Carl
Your relation is 1->1. In these cases I usually return a custom class to the grid that has all the columns I need, including joins with other tables.
So basically what you need is to create a custom linq query with your resultset.
The mais query should follow this example:
var q = from i in ctx.Invoices
join c in ctx.Customers on i.CustomerID equals c.CustomerID
select new{InvoiceID=i.InvoiceID, InvoiceDate=i.Date, CustomerName=c.Name};
Now, assuming we receive a CustomerName variable with the string to filter by c.Name we could do:
if(!string.IsNullOrEmpty(CustomerName))
{
q = q.where(c => c.Name.ToLower().Contains(CustomerName.ToLower()));
}
Notice that I'm performing a ToLower() operation and a Contains, this will beahave as a LIKE ingnoring case sensitivity and searching for the string anywhere in the Customer Name.
At the end you'll return the q.ToList() serialized for the jqGrid...
Suppose I have
table Person
table Employee, which inherits Person.
I want to get a list of Person, regardless if this Person is Employee or not. How do you get entity framework to do that without joining the Employee table? C# is what I'm using. Thanks.
You need to make sure that you don't implicitly use any information from Employee.
If you do this:
var people = Context.People.ToList();
... Then me Entity Framework will initialize new instances of type Employee for those people who happen to be employees. This is because when you store an Employee in the database, you generally expect to get an Employee back when you select the same item.
Nor, for that matter, can you tell the Entity Framework to give you a Person when the stored value is an Employee. The Entity Framework will never give you back an incorrect entity type.
However, if you do just want to read the one table, there is a way to do that: Select the data from Person into a non-entity types, such as an anonymous type:
var people = Context.People.Select(p => new { Name = p.Name }).ToList();