I'm experimenting with db4o as a data store, so to get to grips with it I thought I'd build myself a simple issue tracking web application (in ASP.NET MVC). I've found db4o to be excellent in terms of rapid development, especially for small apps like this, and it also negates the need for an ORM.
However, having come from a SQL Server/MySQL background I'm a little unsure of how I should be structuring my objects when it comes to relationships (or perhaps I just don't properly understand the way object databases work).
Here's my simple example: I have just two model classes, Issue and Person.
public class Issue
{
public string ID { get; set; }
public string Title { get; set; }
public string Description { get; set; }
public DateTime? SubmittedOn { get; set; }
public DateTime? ResolvedOn { get; set; }
public Person AssignedBy { get; set; }
public Person AssignedTo { get; set; }
}
public class Person
{
public string ID { get; set; }
public string Name { get; set; }
public string Email { get; set; }
public string Password { get; set; }
}
The ID properties are just GUID strings generated by the .NET Guid.NewGuid() helper.
So here's how I initially thought the application would work; please ignore any security concerns etc and assume we already have a few Person objects stored in the database:
User logs in. Query the database for the Person which matches the username and password, and store his/her GUID id as a session variable. Redirect to app home screen.
Logged in user creates a new issue ticket, selecting the user to assign it to from a drop-down list. They fill in the other details (Title, Description etc), and then submit the form.
Query the Person objects in the database (by their GUID ID's) to get an object representing the logged in user and one representing the user the ticket has been assigned to. Create a new Person object (populated with the posted form data), assign the Person objects to the Issue object's AssignedBy and AssignedTo properties, and store it.
This would mean I have two Person objects stored against each Issue record. But what happens if I update the original Person—do all the stored references to that Person in the various issue objects update, or do I have to handle that manually? Are they references, or copies?
Would it be better/more efficient to just store a GUID string for the AssignedBy and AssignedTo fields (as below) and then look up the original person based on that each time?
public class Issue
{
public string ID { get; set; }
public string Title { get; set; }
public string Description { get; set; }
public DateTime? SubmittedOn { get; set; }
public DateTime? ResolvedOn { get; set; }
public string AssignedByID { get; set; }
public string AssignedToID { get; set; }
}
I think I'm just stuck in a certain way of thinking which is confusing me. If someone could explain it clearly that would be most helpful!
Object-Databases try to provide the same semantics as objects in memory. The rule of thumb is: It works like objects in memory. Object databases store references between the objects in the database. When you update the object, that object is updates. And if you have a reference to that objects, you see the changed version.
In your case, the Issue-objects refer to the person object. When you update that person, all Issues which refer to it 'see' that update.
Of course, primitive types like int, strings, longs etc are handled like value objects and not a reference objects. Also arrays are handled like value objects in db4o, this means a array is stored together with the object and not as a reference. Everything else is stored as a reference, even collections like List or Dictionaries.
Please take a look at:
http://developer.db4o.com/Documentation/Reference/db4o-7.4/java/reference/html/reference/basic_concepts/database_models/object-relational_how_to.html
Best!
Related
I'm working on a project that want's to control data access in a multi-tenant system. I've got a table set up which has a row on it that says what tenant the object applies to. Let's call this property
ClientObject.ClientOrgId
I want to set something up so that anytime this table is accessed the only results that are returned are results that match some piece of data in the users session. I.e.
ClientObject.ClientOrgId == UserSession.ClientOrgId
and I ideally want to do this restriction on the table model instead of re-implementing it for every query created.
I've found the Autofilter attribute in the service stack documentation, and it looks like the thing that I want to use, but I've been unable to get it working. An example of my code is below, and I'm not seeing any filtering whenever I set the user sessions ClientOrgID to anything different.
[Authenticate]
[Route("/clientObject", HttpMethods.Post)]
[Api("Creates a Client Object")]
public class CreateClientObject : ICreateDb<ClientObjectTableModel>, IReturn<ClientObjectMutationResponse>
{
[ValidateNotEmpty]
public string ClientName{ get; set; }
[ValidateNotEmpty]
public string ClientLocation { get; set; }
[ValidateNotEmpty]
[ValidateNotNull]
public Guid? ClientOrgId { get; set; }
}
[AutoFilter(QueryTerm.Ensure, nameof(ClientObjectTableModel.ClientOrgId), Eval= "userSession.ClientOrgId")]
public class ClientObjectTableModel : AuditBase
{
[AutoId]
public Guid Id { get; set; }
[Required]
public string ClientName { get; set; }
[Required]
public string ClientLocation { get; set; }
[Required]
public Guid ClientOrgId { get; set; }
}
I even went off the rails and tried something like
[AutoFilter(QueryTerm.Ensure, nameof(ClientObjectTableModel.ClientLocation), Value = "The Fourth Moon Of Mars")]
with the expectation that nothing would get returned, and yet I'm still seeing results.
All AutoQuery CRUD Attribute like [AutoFilter] should be applied to the AutoQuery Request DTO, not the data model.
Have a look at how to populate Tenant Ids with AutoPopulate and how it's later used to filter results with [AutoFilter].
I am trying to create a quick demo shop, and I am failing with an optional many to one or zero relationship.
The relevant classes are:
Item
public class Item
{
public int ID { get; set; }
public string Name { get; set; }
public int SubCategoryID { get; set; }
public virtual SubCategory Category { get; set; }
public double Price { get; set; }
}
Order
public class Order
{
public int ID { get; set; }
public DateTime DateOfOrder { get; set; }
public virtual ICollection<Item> Items { get; set; }
public int CustomerID { get; set; }
public virtual Customer Customer { get; set; }
}
However, I am getting confused because viewing the model shows:
but, the database itself shows (for items):
Which indicates to me that each item can only belong to a single order.
Do I have to create a separate class that is many to many orders/items?
I seem to remember EF doing this automatically, but, I haven't touched it for a few years and I just can't remember what I used to do.
I had to add:
public virtual ICollection<order> Orders { get; set; }
to the Item... I'm never going to call it this way, but it looks like that is required for EF to build this relationship.
I am sure it used to be easier, so, leaving this question open so someone can give a better answer!
If you add a collection of Orders to the Item entity, EF will create for you implicitly the junction table on your DB. In this page you can find more info about how to configure a many to many relationship.
The junction table generally is mapped when you need to add an additional column (that excludes both keys of the tables you are joining). In that case you need to create two one-to-many relationships between the entity that represent the junction table and Order and Item respectively. Some people recommend always map the junction table because that way you have all the tables represented as entities and you can write queries starting by the junction table. You can find interesting info about this subject in this link. But, in my experience, in most cases you can work perfectly without map explicitly the junction table.
I'm all new to databases and I've heard that SQLite should be straight forward, but I kinda find that not to be the case.
I have a decent knowledge of C# and WPF and I'm trying to make a small personal project which I want to use in daily life. My idea is a program that manages receipts that I bring home.
I have two Models - Item and Receipt
class Item
{
public int Id { get; set; }
public string Name { get; set; }
public double Price { get; set; }
public int ReceiptId { get; set; }
}
class Receipt
{
public int Id { get; set; }
public string Store { get; set; }
public double Price { get; set; }
public DateTime Date { get; set; }
}
My corrent problem is that I've been caught in all the different SQLite "versions" available. I've found that there is ADO.NET and LINQ and some other sorts.. what does these different ones mean and is the programming syntax for each of them different and which would be most suitable for my case?
I'm developing in Visual Studio 2012 and I've managed to download the NuGet SQLite package that goes by the name "System.Data.SQLite (x86/x64)".
I want to create a database, create two tables with these object in them, but I have basicly no idea how to and searching google for a day or two has just me that more confused, since so many people use different syntaxes and most of the tutorials I've find relate to how people used the old "SQLite.dll". So what my question is, is how do I make a database that fits this program structure and how do I write, read, connect and disconnect to my database.
Document oriented databases (particularly RavenDB) are really intriguing me, and I'm wanting to play around with them a bit. However as someone who is very used to relational mapping, I was trying to think of how to model data correctly in a document database.
Say I have a CRM with the following entities in my C# application (leaving out unneeded properties):
public class Company
{
public int Id { get; set; }
public IList<Contact> Contacts { get; set; }
public IList<Task> Tasks { get; set; }
}
public class Contact
{
public int Id { get; set; }
public Company Company { get; set; }
public IList<Task> Tasks { get; set; }
}
public class Task
{
public int Id { get; set; }
public Company Company { get; set; }
public Contact Contact { get; set; }
}
I was thinking of putting this all in a Company document, as contacts and tasks do not have a purpose out side of companies, and most of the time query for a task or contacts will also show information about the associated company.
The issue comes with Task entities. Say the business requires that a task is ALWAYS associated with a company but optionally also associated with a task.
In a relational model this is easy, as you just have a Tasks table and have the Company.Tasks relate to all tasks for the company, while Contact.Tasks only show the tasks for the specific Task.
For modeling this in a document database, I thought of the following three ideas:
Model Tasks as a separate document. This seems kind of anti-document db as most of the time you look at a company or contact you will want to see the list of tasks, thus having to perform joins over documents a lot.
Keep tasks that are not associated with a contact in the Company.Tasks list and put tasks assocaited with a contact in the list for each individual contacts. This unfortunately means that if you want to see all tasks for a company (which will probably be a lot) you have to combine all tasks for the company with all tasks for each individual contact. I also see this being complicated when you want to disassociate a task from a contact, as you have to move it from the contact to the company
Keep all tasks in the Company.Tasks list, and each contact has a list of id values for tasks it is associated with. This seems like a good approach except for having to manually take id values and having to make a sub-list of Task entities for a contact.
What is the recommended way to model this data in a document oriented database?
Use denormalized references:
http://ravendb.net/faq/denormalized-references
in essence you have a DenormalizedReference class:
public class DenormalizedReference<T> where T : INamedDocument
{
public string Id { get; set; }
public string Name { get; set; }
public static implicit operator DenormalizedReference<T> (T doc)
{
return new DenormalizedReference<T>
{
Id = doc.Id,
Name = doc.Name
}
}
}
your documents look like - i've implemented the INamedDocument interface - this can be whatever you need it to be though:
public class Company : INamedDocument
{
public string Name{get;set;}
public int Id { get; set; }
public IList<DenormalizedReference<Contact>> Contacts { get; set; }
public IList<DenormalizedReference<Task>> Tasks { get; set; }
}
public class Contact : INamedDocument
{
public string Name{get;set;}
public int Id { get; set; }
public DenormalizedReference<Company> Company { get; set; }
public IList<DenormalizedReference<Task>> Tasks { get; set; }
}
public class Task : INamedDocument
{
public string Name{get;set;}
public int Id { get; set; }
public DenormalizedReference<Company> Company { get; set; }
public DenormalizedReference<Contact> Contact { get; set; }
}
Now saving a Task works exactly as it did before:
var task = new Task{
Company = myCompany,
Contact = myContact
};
However pulling all this back will mean you're only going to get the denormalized reference for the child objects. To hydrate these I use an index:
public class Tasks_Hydrated : AbstractIndexCreationTask<Task>
{
public Tasks_Hydrated()
{
Map = docs => from doc in docs
select new
{
doc.Name
};
TransformResults = (db, docs) => from doc in docs
let Company = db.Load<Company>(doc.Company.Id)
let Contact = db.Load<Contact>(doc.Contact.Id)
select new
{
Contact,
Company,
doc.Id,
doc.Name
};
}
}
And using your index to retrieve the hydrated tasks is:
var tasks = from c in _session.Query<Projections.Task, Tasks_Hydrated>()
where c.Name == "taskmaster"
select c;
Which i think is quite clean :)
As a design conversation - the general rule is that if you ever need to load the child documents alone as in - not part of the parent document. Whether that be for editing or viewing - you should model it with it's own Id as it's own document. Using the method above makes this quite simple.
I'm new to document dbs as well...so with a grain of salt...
As a contrasting example...if you are on Twitter and you have a list of the people you follow, which contains a list of their tweets...you would not move their tweets into your twitter account in order to read them, and if you re-tweet, you would only have a copy, not the original.
So, in the same way, my opinion is that if Tasks belong to a company, then they stay within the Company. The Company is the Aggregate Root for Tasks. The Contacts can only hold references (ids) or copies of the Tasks and cannot modify them directly. If you have your contact hold a "copy" of the task, that's fine, but in order to modify the task (e.g. mark it complete) you would modify the task through its Aggregate Root (Company). Since a copy could quickly become outdated, it seems like you would only want a copy to exist while in memory and when saving the Contact, you would only save references to the Tasks.
I have a usercontrol which is responsible for presenting creation and change of users.
The usercontrol is bound to an entity delivered by a RIA Service:
[MetadataType(typeof(User.UserMetadata))]
public partial class User
{
internal class UserMetadata
{
protected UserMetadata() {}
[Required]
public string Name { get; set; }
[Exclude]
public string PasswordHash { get; set; }
[Exclude]
public int PasswordSalt { get; set; }
[Required]
public string ShortName { get; set; }
[Include]
public IEnumerable<UserRole> UserRoles { get; set; }
}
[DataMember]
[RegularExpression("^.*[^a-zA-Z0-9].*$", ErrorMessageResourceName = "BadPasswordStrength", ErrorMessageResourceType = typeof(ErrorResources))]
[StringLength(25, MinimumLength = 6)]
public string NewPassword { get; set; }
}
When creating a new user, the field "NewPassword" is required - but when changing properties of an existing user, it is not (it is used for password-changes).
What is the best approach to solve this? I have several ideas, but they all feels a little bit crappy :-)
Thanks
It appears you are passing your current passwords back to the GUI. There is no need to ever do that. That would create a potential security hole
Suggest you treat password changing as a separate service call, not just a simple record editing exercise. RIA services supports Invoke Operations which are basically arbitrary direct-calls to your RIA service. The only restriction on Invoke operations is that they cannot return complex types (not a problem for this example).
Pass your current logged-in user identifier, the current password (encoded) and the new password (encoded) and do all the work server side. Return a simple success boolean value.
Just some suggestions, I am happy to see other people's ideas on this one :)