Added the selection processor for adding & loading data

I18N Commander/Processor/SectionProcessor.cs

@@ -0,0 +1,74 @@
+using DataModel.Database;
+using DataModel.Database.Common;
+using Microsoft.EntityFrameworkCore;
+
+namespace Processor;
+
+public static class SectionProcessor
+{
+    /// <summary>
+    /// Load one layer of the tree by using the specified depth:
+    /// </summary>
+    public static IAsyncEnumerable<Section> LoadLayer(DataContext db, int depth)
+    {
+        return db.Sections.Where(n => n.Depth == depth).OrderBy(n => n.DataKey).AsAsyncEnumerable();
+    }
+
+    /// <summary>
+    /// Determine how deep the tree is.
+    /// </summary>
+    public static async ValueTask<int> GetDepth(DataContext db)
+    {
+        if(!await db.Sections.AnyAsync())
+        {
+            return 0;
+        }
+
+        return await db.Sections.MaxAsync(s => s.Depth);
+    }
+    
+    /// <summary>
+    /// Compute the new sections key and its depth, then store the section in the database.
+    /// </summary>
+    public static async Task<Section> AddSection(DataContext db, string text, string? parentKey)
+    {
+        // Remove any whitespaces from the section name, regardless of how many e.g. spaces the user typed:
+        var key = string.Join('_', text.Split(' ', StringSplitOptions.TrimEntries | StringSplitOptions.RemoveEmptyEntries)).ToUpperInvariant();
+
+        // In the case, when the user adds a section to the root, handle the insert differently:
+        if (string.IsNullOrEmpty(parentKey))
+        {
+            var rootSection = new Section
+            {
+                Depth = 0,
+                DataKey = key,
+                Parent = null,
+                Name = text.Trim(),
+                TextElements = new(),
+            };
+
+            db.Sections.Add(rootSection);
+            await db.SaveChangesAsync();
+            return rootSection;
+        }
+
+        // Read the parent from the database:
+        var parent = await db.Sections.FirstOrDefaultAsync(n => n.DataKey == parentKey);
+        if (parent is null)
+            throw new ArgumentException($"The section's parent with key {parentKey} does not exist in the database.");
+
+        // Add the new section to the database:
+        var section = new Section
+        {
+            Name = text.Trim(),
+            DataKey = key,
+            Parent = parent,
+            TextElements = new(),
+            Depth = parent.Depth + 1,
+        };
+        
+        db.Sections.Add(section);
+        await db.SaveChangesAsync();
+        return section;
+    }
+}