📓 3.2.2.4 Finding Objects with Unique IDs
We'll eventually want users to click an individual Item in our To Do List to view its details on a special detail page. To do this, our application needs to differentiate between Items. Otherwise, the server won't know which Item's details to display. That means we need to add a unique ID to our Item model. Then we can use routing to differentiate between Items like this: items/1 and items/2.
Assigning Unique IDs
Testing
Because this requires updating our Item.cs model logic, we'll need to add a test.
First, however, let's run our previous tests by navigating to the test directory and running dotnet test.
All 5 should pass, but if there are errors, try running dotnet restore in the test directory.
Let's add our new test:
...
[TestMethod]
public void GetId_ItemsInstantiateWithAnIdAndGetterReturns_Int()
{
//Arrange
string description = "Walk the dog.";
Item newItem = new Item(description);
//Act
int result = newItem.Id;
//Assert
Assert.AreEqual(1, result);
}
...
Our new test:
Arranges the necessary materials by creating a sample
Item;Calls a new
Idproperty from theItem;Asserts that
Idreturns theint1.
Implementing Unique IDs
Because our new test references an Id property that doesn't exist yet, we won't be able to run our tests. In order to get around this and see our test fail, however, let's temporarily change the 'Act' portion of our test to this:
int result = 0;
Now we should be able to see our test fail properly.
Let's change the value of the result variable in the test back to newItem.Id.
int result = newItem.Id;
Let's also add our logic to make this test pass. We'll begin by updating our Item class to contain a new Id property:
...
public class Item
{
public string Description { get; set; }
public int Id { get; }
private static List<Item> _instances = new List<Item> {};
...
Then we'll set this property in the constructor:
...
public Item(string description)
{
Description = description;
_instances.Add(this);
Id = _instances.Count;
}
...
...
...
This assigns an Item's Id to the current number of Items in the static _instances list. After the first Item is added, _instances.Count will be 1. After the second Item, it'll be 2, and so on. Using _instances.Count to assign Id ensures each is always unique. Note that we do this after adding Items to the _instances list in order to get an updated Count for Id.
One more thing: Remember that when we are working inside an object, we can use the keyword this to reference that object. In the code above, we use this to reference the Item being actively constructed by the constructor.
We don't add a set method, because this property will be set in the constructor automatically. In fact, we specifically don't ever want to manually edit it. That would increase the risk of IDs not being unique.
This is called creating a readonly property. In other words, it's a property that can be read but not overwritten.
...
public int Id { get; }
...
If we run our tests again, they'll all pass.
Finding Items By ID
Next, we'll need to locate specific Items based on their new unique Ids. This will help our server populate our eventual Item detail pages with the correct object's information.
Testing
Let's write a test for this functionality:
...
[TestMethod]
public void Find_ReturnsCorrectItem_Item()
{
//Arrange
string description01 = "Walk the dog";
string description02 = "Wash the dishes";
Item newItem1 = new Item(description01);
Item newItem2 = new Item(description02);
//Act
Item result = Item.Find(2);
//Assert
Assert.AreEqual(newItem2, result);
}
...
This test arranges multiple sample
Itemobjects.It then calls a static
Find()method, passing in the int2as an argument because this is the anticipatedIdproperty of the secondItem.We then assert that
Find()will returnnewItem2when provided the argument2.
Properly Failing the Test
Similar to when we added our last test, we won't be able to compile and run our test suite because we are referencing a method that doesn't yet exist. In order to properly fail this test, we'll have to set the value of result to a different Item. Normally, we could just set the value of result to 0, but this time, because result is of type Item, we'll set it to a new Item we know will give us a proper failure.
Let's do that now:
...
//Act
Item result = new Item("Incorrect test item");
...
If we run dotnet test we can now see that our test is failing as expected. Let's now reset the value of result.
...
//Act
Item result = Item.Find(2);
...
Implementing Find()
Let's add logic for our new static Find() method:
...
public static Item Find(int searchId)
{
return _instances[searchId-1];
}
...
We create a static method to locate an Item using its Id. It's static because it must sift through all Items to find the one we're seeking. And because it's static, we must call it on the entire Item class, as seen in the test above.
Also, notice we subtract 1 from the provided searchId because indexes in the _instances array begin at 0, whereas our Id properties will begin at 1.
The entire updated Item.cs model file looks like this:
using System.Collections.Generic;
namespace ToDoList.Models
{
public class Item
{
public string Description { get; set; }
public int Id { get; }
private static List<Item> _instances = new List<Item> { };
public Item(string description)
{
Description = description;
_instances.Add(this);
Id = _instances.Count;
}
public static List<Item> GetAll()
{
return _instances;
}
public static void ClearAll()
{
_instances.Clear();
}
public static Item Find(int searchId)
{
return _instances[searchId - 1];
}
}
}
Try running tests and verify that they pass. We can now automatically assign each Item a guaranteed unique Id that can be used to locate specific objects. In the next few lessons, we'll add functionality for Item detail pages to our app.
Repository Reference
Follow the link below to view how a sample version of the project should look at this point. Note that this is a link to a specific branch in the repository.
Example GitHub Repo for To Do List: 5_deleting_and_finding_objects