Add documentation for database configurations in Single Layer Web Application template

docs/en/solution-templates/single-layer-web-application/database-configurations.md

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+# Single Layer Solution: Database configurations
+
+```json
+//[doc-nav]
+{
+  "Previous": {
+    "Name": "Authentication",
+    "Path": "solution-templates/single-layer-web-application/authentication"
+  },
+  "Next": {
+    "Name": "Logging (with Serilog)",
+    "Path": "solution-templates/single-layer-web-application/logging"
+  }
+}
+```
+
+> Some of the features mentioned in this document may not be available in the free version. We're using the **\*** symbol to indicate that a feature is available in the **[Team](https://abp.io/pricing)** and **[Higher](https://abp.io/pricing)** licenses.
+
+ABP Studio's Single-Layer Solution Template includes pre-configured database settings. This document explains how to manage database configurations in your solution.
+
+## Connection String
+
+Connection strings are stored in the `appsettings.json` file and can be customized for different environments by editing this file. [Web Application](web-applications.md) projects use the `Default` connection string by default.
+
+To update the connection string for the `Default` key, modify the `appsettings.json` file in your project. Connection strings are defined under the `ConnectionStrings` section, as shown below:  
+
+```json
+{
+  "ConnectionStrings": {
+    "Default": "Server=(LocalDb)\\MSSQLLocalDB;Database=Bookstore;Trusted_Connection=True;TrustServerCertificate=true"
+  }
+}
+```
+
+## The DbContext Class
+
+In the Single-Layer Solution Template, the `DbContext` class is defined in the main project. This class manages the database schema and is derived from the `AbpDbContext` class, which offers additional features and configurations. You can customize the `DbContext` class to add new entities, relationships, and configurations. It is located in the `Data` folder of the main project.  
+
+```csharp
+public class BookstoreDbContext : AbpDbContext<BookstoreDbContext>
+{
+    
+    public const string DbTablePrefix = "App";
+    public const string DbSchema = null;
+
+    public BookstoreDbContext(DbContextOptions<BookstoreDbContext> options)
+        : base(options)
+    {
+    }
+
+    protected override void OnModelCreating(ModelBuilder builder)
+    {
+        base.OnModelCreating(builder);
+
+        /* Include modules to your migration db context */
+
+        builder.ConfigureSettingManagement();
+        builder.ConfigureBackgroundJobs();
+        builder.ConfigureAuditLogging();
+        builder.ConfigureFeatureManagement();
+        builder.ConfigurePermissionManagement();
+        builder.ConfigureBlobStoring();
+        builder.ConfigureIdentityPro();
+        builder.ConfigureOpenIddictPro();
+        builder.ConfigureGdpr();
+        builder.ConfigureLanguageManagement();
+        builder.ConfigureSaas();
+        builder.ConfigureTextTemplateManagement();
+        
+        /* Configure your own entities here */
+    }
+}
+```
+
+### OnModelCreating Method
+
+The `OnModelCreating` method is used to configure the database schema. It calls the `Configure*` methods of the ABP Framework to configure the database schema for the modules. You can also configure your own tables/entities inside this method.
+
+```csharp
+protected override void OnModelCreating(ModelBuilder builder)
+{
+    base.OnModelCreating(builder);
+
+    builder.ConfigurePermissionManagement();
+    builder.ConfigureSettingManagement();
+    builder.ConfigureBackgroundJobs();
+    builder.ConfigureAuditLogging();
+    builder.ConfigureFeatureManagement();
+    builder.ConfigureIdentityPro();
+    builder.ConfigureOpenIddictPro();
+    builder.ConfigureLanguageManagement();
+    builder.ConfigureSaas();
+    builder.ConfigureTextTemplateManagement();
+    builder.ConfigureGdpr();
+    builder.ConfigureCmsKit();
+    builder.ConfigureCmsKitPro();
+    builder.ConfigureBlobStoring();
+    
+    /* Configure your own tables/entities inside here */
+
+    //builder.Entity<YourEntity>(b =>
+    //{
+    //    b.ToTable(DbTablePrefix + "YourEntities", DbSchema);
+    //    b.ConfigureByConvention(); //auto configure for the base class props
+    //    //...
+    //});
+}
+```
+
+> The `Configure*` methods are extension methods defined in each module's `EntityFrameworkCore` project. These methods are used to configure the database schema for their respective modules.
+
+## The `IDesignTimeDbContextFactory` Implementation
+
+The `IDesignTimeDbContextFactory` interface is used to create a `DbContext` instance at design time. It is used by EF Core tools to create migrations and update the database. The `BookstoreDbContextFactory` class implements the `IDesignTimeDbContextFactory` interface to create a `BookstoreMigrationsDbContext` instance.
+
+```csharp
+public class BookstoreDbContextFactory : IDesignTimeDbContextFactory<BookstoreDbContext>
+{
+    public BookstoreDbContext CreateDbContext(string[] args)
+    {
+        BookstoreEfCoreEntityExtensionMappings.Configure();
+        var configuration = BuildConfiguration();
+
+        var builder = new DbContextOptionsBuilder<BookstoreDbContext>()
+            .UseSqlServer(configuration.GetConnectionString("Default"));
+
+        return new BookstoreDbContext(builder.Options);
+    }
+
+    private static IConfigurationRoot BuildConfiguration()
+    {
+        var builder = new ConfigurationBuilder()
+            .SetBasePath(Directory.GetCurrentDirectory())
+            .AddJsonFile("appsettings.json", optional: false);
+
+        return builder.Build();
+    }
+}
+```
+
+## Configuration
+
+In the `BookstoreModule` class, the `ConfigureEfCore` method is used to configure the database context. It registers the `BookstoreDbContext` class to the [dependency injection](../../framework/fundamentals/dependency-injection.md) system and sets the SQL Server as the default DBMS for the application.
+
+```csharp
+private void ConfigureEfCore(ServiceConfigurationContext context)
+{
+    context.Services.AddAbpDbContext<BookstoreDbContext>(options =>
+    {
+        /* You can remove "includeAllEntities: true" to create
+            * default repositories only for aggregate roots
+            * Documentation: https://docs.abp.io/en/abp/latest/Entity-Framework-Core#add-default-repositories
+            */
+        options.AddDefaultRepositories(includeAllEntities: true);
+    });
+
+    Configure<AbpDbContextOptions>(options =>
+    {
+        options.Configure(configurationContext =>
+        {
+            configurationContext.UseSqlServer();
+        });
+    });
+    
+}
+```
+
+### SaaS Module: The Tenant Management UI **\***
+
+SaaS module provides the necessary UI to set and change connection string for tenants and trigger the database migrations.
+
+#### The Connection String Management Modal
+
+You can click to the *Database Connection Strings* command in the *Actions* dropdown button for a tenant in the *Tenants* page of the SaaS module: