Add doc for database migrates.

Author: TheNexusAvenger

docs/database-schema-changes.md

@@ -0,0 +1,87 @@
+# Database Schema Changes
+This document covers making changes to the database schema (how the database tables
+are set up) in Uchu in development. Readng and writing to the database while running
+is not covered.
+
+## Setup
+Part of the setup for pushing changes requires making database migrates, which defines
+how a database migrates to new versions. To start, make sure you have `dotnet` in
+your system PATH and either `python3` or `python` in your PATH. Which Python keyword
+you use depends on your setup, but it will most likely be `python` on Windows and
+`python3` on other systems. See external instructions for your operating system for
+setting up .NET and Python.
+
+With .NET installed, you will also need the Microsoft Entity Framework CLI tools.
+This only needs to be run once if you don't have the CLI tools installed:
+```bash
+dotnet tool install --global dotnet-ef
+```
+
+## Making Schema Changes
+Since Uchu uses Microsoft Entity Framework, changes to the database schema are done
+in the C# classes. No matter what is done, database migrates will be required.
+
+### Column Changes
+The simpler of the two types of changes are to the columns of an existing table/class,
+including adding or deleting (mostly adding - be careful deleting data for backward
+compatibility). To do this, modify the database model class in [Uchu.Core/Database/Models](../Uchu.Core/Database/Models).
+Properties in the models can also have attributes. Some examples include:
+* `[Key]` - Marks the column as being the primary key. *1 and only 1 column must have this.*
+* `[Required]` - Marks the column as being required, or `NOT NULL` in SQL.
+* `[MaxLength(N)]` - For strings, this specifies the max length of strings as length N.
+
+With Entity Framework, some types are able to be automatically handled, such as `DateTime`s
+and references to other database models.
+
+### New Models/Tables
+Adding new database models/tables is more complicated since it requires creating the new
+database model and setting it up with the database contexts. The steps to do this involve:
+1. Create the new database model as a C# class in [Uchu.Core/Database/Models](../Uchu.Core/Database/Models).
+   *Remember to specify the primary key with `[Key]`.*
+2. Add the Model as a `DbSet<ClassName>` in [Uchu.Core/Database/Providers/UchuContextBase.cs](../Uchu.Core/Database/Providers/UchuContextBase.cs)
+   similar to the other `DbSet<T>` properties.
+3. Add the Model as a `DbSet<ClassName>` in [Uchu.Core/Database/UchuContext.cs](../Uchu.Core/Database/UchuContext.cs)
+   similar to the other `DbSet<T>` properties.
+
+## Creating Database Migrates
+After the changes to the schema have been made, database migrates need to be made so the
+database can be updated when Uchu starts to have the new columns or tables. Both ways
+require a unique name for the migration. While it can be random, it is recommended to
+be a human-readable name relevant to the change. In the commands below, this will be
+`MigrateName`.
+
+### Script (Recommended)
+*If you are on macOS, this method is not supported since it involves downloading MariaDB,*
+*which does not have downloads for macOS. Contributions are open for resolving this.*
+
+A helper script exists for creating the database migrates for all the supported database
+providers. The following can be run to generate the database migrates:
+```bash
+cd scripts
+python3 CreateMigrates.py MigrateName
+```
+
+As mentioned before, `python3` may need to be replaced with `python`.
+
+### Manually (Not Recommended)
+Unlike SQLite and PostgreSQL, MySQL requires a running database. Setup on how to do
+this is not covered and it is recommended to perform a proper secure setup.
+
+For SQLite and PosgreSQL, the migrations can be created with the following:
+```bash
+cd Uchu.Core
+dotnet ef migrations add SqliteMigrationName --context SqliteContext
+dotnet ef migrations add PostgresMigrationName --context PostgresContext
+```
+
+The MySQL step is similar, but you will need to modify [Uchu.Core/Database/Providers/MySqlContext.cs](../Uchu.Core/Database/Providers/MySqlContext.cs)
+temporarily with a valid connection string for your setup. **Make sure to
+not commit this and to revert it.** After this, run the following:
+```bash
+dotnet ef migrations add MySqlMigrationName --context MySqlContext
+```
+
+## Commit Changes
+After creating or modifying the database models, modifying the database contexts
+(if applicable), and generating the migrates, the changes can be committed including
+after trying to start Uchu. An error should occur if the migrates cause issues.