Read

We have our documents stored, but now we want to read them! Let's start with two common operations:

• findAll()
• findById()

findAll

To retrieve all the grades form the database, all you need is the findAll() method provided by the StudentRepository. This method will return all the grade documents present in the grades collection.

public List<Grade> getAllGrades() {
    return repository.findAll();
}

findById

To retrieve individual documents, you can use the findById() method provided by the StudentRepository. This allows you to retrieve an individual document from the database, identified by the ObjectId.

public Optional<Grade> getGradeById(String id) {
    return repository.findById(id);
}

Query Generation

While these methods are powerful as they are, it only touches on the capabilities of MongoRepository. Query generation allows us to build queries solely based of the method names. If we look back on our document model for this collection;

{
  "_id": ObjectId,
  "studentId": double,
  "classId": double,
  "scores": [
    {
      "type": String,
      "score": double
    }
  ],
  "comment": String
}

Imagine if you wanted to search by comment. With MongoRepository, all you have to do is create a method called findByComment(String comment) and it will generate a query to find documents with a specific comment.

There are a number of ways you can adjust these methods, such as paging, iterating large results, sorting & limiting, and if you want to read about the nitty gritty of each of these, head over to the Spring Documentation.

In simple terms, When creating query method names, they are split into two parts: the subject and the predicate.

1. Subject: The first part (like findBy or existsBy) tells what the query will do.
2. Predicate: The second part describes the conditions of the query.

The subject can include additional words that describe the query. Any words between find (or other starting keywords) and By are just for description, unless they are special keywords like Distinct (to ensure unique results) or Top/First (to limit the number of results). You can see what each word maps to in the Spring documentation, and it is largely intuitive.

For example, in the method findTop3ByAge, findTop3 is the subject, and ByAge is the predicate. The Top3 indicates that the query should return the first three results based on the Age condition.