finished data model description

Rewritten-docs.md

@@ -3193,7 +3193,7 @@ this "concrete" assignment, such as the name, version and a private description.
 
 Some exercise descriptions need to be translated into multiple languages.
 Because of this, the `Exercise` entity is associated with the
-`LocalizedAssignment` entity, one for each translation of the text. 
+`LocalizedText` entity, one for each translation of the text. 
 
 An exercise can support multiple programming runtime environments. These
 environments are represented by `RuntimeEnvironment` entities. Apart from a name
@@ -3202,7 +3202,7 @@ is being used. There is also a list of extensions that is used for detecting
 which environment should be used for student submissions.
 
 `RuntimeEnvironment` entities are not linked directly to exercises. Instead,
-the `Exercise` entity has an M:N relation with the `SolutionRuntimeConfig`,
+the `Exercise` entity has an M:N relation with the `RuntimeConfig` entity,
 which is associated with `RuntimeEnvironment`. It also contains a path to a job
 configuration file template that will be used to create a job configuration file
 for the worker that processes solutions of the exercise.
@@ -3222,6 +3222,10 @@ entities can have multiple `ReferenceSolutionEvaluation` entities associated
 with them that link to evaluation results (`SolutionEvaluation` entity). Details
 of this structure will be described in the section about student solutions.
 
+Source codes of the reference solutions can be accessed using the `Solution`
+entity associated with `ReferenceSolution`. This entity is also used for student
+submissions.
+
 #### Assignments
 
 The `Assignment` entity is created from an `Exercise` entity when an exercise is
@@ -3230,16 +3234,51 @@ reference documentation for a detailed overview). Additional information such as
 deadlines or point values for individual tests is also configured for the
 assignment and not for an exercise.
 
-Assignments can also have their own `LocalizedAssignment` entities. If the
+Assignments can also have their own `LocalizedText` entities. If the
 assignment texts are not changed, they are shared between the exercise and its
 assignment.
 
 Runtime configurations can be also changed for the assignment. This way, a
-supervisor can for example alter the resource limits for the tests.
+supervisor can for example alter the resource limits for the tests. They could
+also alter the way submissions are evaluated, which is discouraged.
 
 #### Student Solutions
 
-#### Comment threads
+Solutions submitted by students are represented by the `Submission` entity. It
+contains data such as when and by whom was the solution submitted. There is also
+a timestamp, a note for the supervisor and an url of the location where
+evaluation results should be stored. 
+
+However, the most important part of a submission are the source files. These are
+stored using the `SolutionFile` entity and they can be accessed through the
+`Solution` entity, which is associated with `Submission`.
+
+When the evaluation is finished, the results are stored using the
+`SolutionEvaluation` entity. This entity can have multiple `TestResult` entities
+associated with it, which describe the result of a test and also contain
+additional information for failing tests (such as which limits were exceeded).
+Every `TestResult` can contain multiple `TaskResult` entities that provide
+details about the results of individual tasks. This reflects the fact that
+"tests" are just logical groups of tasks.
+
+#### Comment Threads
+
+The `Comment` entity contains the author of the comment, a date and the text of
+the comment. In addition to this, there is a `CommentThread` entity associated
+with it that groups comments on a single entity (such as a student submission).
+This enables easily adding support for comments to various entities -- it is
+enough to add an association with the `CommentThread` entity. An even simpler
+way is to just use the identifier of the commented entity as the identifier of
+the comment thread, which is how submission comments are implemented.
+
+#### Uploaded Files
+
+Uploaded files are stored directly on the filesystem instead of in the database.
+The `UploadedFile` entity is used to store their metadata. This entity is
+extended by `SolutionFile` and `ExerciseFile` using the Single Table Inheritance
+pattern provided by Doctrine. Thanks to this, we can access all files uploaded
+on the API through the same repository while also having data related to e.g.,
+supplementary exercise files present only in related objects.
 
 ### Request Handling