Skip to content

Exercise: MongoDB

This exercise is optional. You may earn 2 points by completing this exercise.

Use GitHub Classroom to get your git repository. You can find the invitation link in Moodle. Clone the repository created via the link. It contains a skeleton and the expected structure of your submission. After completing the exercises and verifying them, commit and push your submission.

Check the required software and tools here.

Before you begin, create and initialize the database; use the steps the seminar exercises describe.

Exercise 0: Neptun code

Your very first task is to type your Neptun code into neptun.txt in the root of the repository.

Exercise 1: Product with the largest total value (2 points)

The task is to find the product that has the largest total value within a product category. The total value is the price of the product multiplied by the amount of the product in stock. You need to implement the following method in class ProductRepository.

(string, double?) ProductWithLargestTotalValue(ObjectId categoryId)

  1. Let us check the test related to this exercise in file TestExercise1.cs to understand what is expected here.

    • The method accepts a category filter as an argument; products have to be filtered for this category.
    • The return value should be the name of the product (with the largest total value) and the total value itself.
    • If there are no products in the specified category, the return value should be (null, null).

  2. Use the aggregation pipeline of MongoDB. To see how this aggregation pipeline works, you can refer to the seminar material.

    You should build a pipeline consisting of the following stages:

    • Filter the products for the specified category. Use a $match (Match) stage to specify the filter.

    • Calculate for each product the total value (multiply the price and the stock) using a $project (Project) stage. Make sure to include the name of the product, you will need it for the final result.

    • Order the items based on this calculated total value descending. Use a $sort (SortByDescending) stage.

    • Since it is the largest value that we need, take the first item after sorting. Do not forget that there might not be any product in the specified category. Therefore you should use FirstOrDefault to fetch this item.

    If the syntax (string, double?) is unfamiliar:

    return ("test", 0.0);

    The function will return with these two values.

  3. Implement the repository method. The repository class receives the database as a parameter and saves the collection as a local variable in the class; use this field to manipulate the collection.

There are unit tests available in the solution. You can run the unit tests in Visual Studio, or if you are using another IDE (e.g., VS Code or dotnet cli), then run the tests using the cli. You may update the database connection string in class TestDbFactory if needed.

Tests

The tests presume that the database is in its initial state. Re-run the database initialization script to restore this state.

Do NOT change the unit tests. You may temporarily alter the unit tests if you need to, but make sure to reset your changes before committing.

SUBMISSION

Upload the changed C# source code.

You can run the tests in Visual Studio or using dotnet cli. Make sure that the screenshot includes the source code of the repository and the test execution outcome! Save the screenshot as f1.png and upload as part of your submission!

If you are using dotnet cli to run the tests, make sure to display the test names too. Use the -v n command line switch to set detailed logging.

The image does not need to show the exact same source code that you submit; there can be some minor changes. If the tests run successfully and you create the screenshot, then later you make some minor change to the source, there is no need for you to update the screenshot.

Exercise 2 optional: Estimating storage space (0 points)

The company is moving to a new location. We need to know whether the current stock can be moved and will fit into the new storage facility. Implement the method that calculates the total volume of all products in stock!

The products have the necessary information in description.product.package_parameters:

Product size

Use this to calculate the total volume of all items:

  • Use the information from package_parameters (and not from product_size).
  • A product might have multiple packages; this information is available in package_parameters.number_of_packages. This number shall be used as a multiplicator. Each product has a single size, and if it has multiple packages, then all packages are of the same size.
  • The final total: for all products Σ (product stock * number of packages * width * height * depth).
  • If a product does not have these information, it's volume should be calculated as 0.
  • Mind, that the size also has a unit: either cm or m, but the final value is expected in cubic meter.

Implement method double GetAllProductsCumulativeVolume() that returns a single scalar total of the volume in cubic meters. The calculation should be performed by the database (not in C#); use the aggregation pipeline.

Sum aggregation

You will need the $group aggregation stage. Although, we do not need to group the products, still, this will allow us to aggregate all of them. Map each product into the same group (that is, in the $group stage the id should be a constant for all items), then use the projection part to perform a $sum aggregation according to the formula above.

Handling of the cm and m units can be solved with a conditional multiplication in the sum. If this does not work, you can use two aggregations: one for cm and one for m units, each with a filter in the pipeline then the aggregation afterwards.

The required parts of the products are not mapped to C# classes yet. You need to do this. Note, that the field names in the BSON do not conform to the usual syntax, thus, when mapping to C# properties, you have to take care of name the properties identically, or use the [BsonElement(elementName: "...")] attribute.

Use Fluent Api

You must use the C# Fluent Api! Do not write the query using BsonDocument!

You may test your implementation with the tests provided in class TestExercise2. The tests presume that the database is in its initial state.

SUBMISSION

Upload the changed C# source code.

2023-10-09 Contributors