Keep a lone list property's key when merging properties [patch]

Frontmatter.Test/ListKeyNamingTests.cs

@@ -0,0 +1,50 @@
+// Copyright (c) 2023-2026 ktsu-dev contributors
+
+namespace ktsu.Frontmatter.Test;
+
+using System.Collections.Generic;
+
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+/// <summary>
+/// Regression tests for #147: merging must not rename a list property, since renaming is the job of
+/// <see cref="FrontmatterNaming.Standard"/>, and <see cref="FrontmatterNaming.AsIs"/> turns it off.
+/// </summary>
+[TestClass]
+public class ListKeyNamingTests
+{
+	private static readonly string Nl = Environment.NewLine;
+
+	[TestMethod]
+	[DataRow(FrontmatterMergeStrategy.None)]
+	[DataRow(FrontmatterMergeStrategy.Conservative)]
+	[DataRow(FrontmatterMergeStrategy.Aggressive)]
+	[DataRow(FrontmatterMergeStrategy.Maximum)]
+	public void CombineFrontmatter_AsIsNaming_LoneListPropertyKeepsItsKey(FrontmatterMergeStrategy strategy)
+	{
+		string input = $"---{Nl}section:{Nl}  - news{Nl}summary: hi{Nl}---{Nl}Body{Nl}";
+
+		string result = Frontmatter.CombineFrontmatter(input, FrontmatterNaming.AsIs, FrontmatterOrder.AsIs, strategy);
+		Dictionary<string, object>? frontmatter = Frontmatter.ExtractFrontmatter(result);
+
+		Assert.IsNotNull(frontmatter);
+		Assert.IsTrue(frontmatter.ContainsKey("section"), $"section was renamed: {result}");
+		Assert.IsFalse(frontmatter.ContainsKey("categories"), $"section was renamed: {result}");
+	}
+
+	[TestMethod]
+	[DataRow("keywords")]
+	[DataRow("category")]
+	public void MergeSimilarProperties_LoneListProperty_KeepsItsKey(string key)
+	{
+		Dictionary<string, object> source = new()
+		{
+			[key] = new List<object> { "a", "b" },
+		};
+
+		Dictionary<string, object> result = PropertyMerger.MergeSimilarProperties(source, FrontmatterMergeStrategy.Conservative);
+
+		Assert.IsTrue(result.ContainsKey(key), $"{key} was renamed to {string.Join(", ", result.Keys)}");
+		Assert.AreEqual(1, result.Count);
+	}
+}

Frontmatter/PropertyMerger.cs

@@ -93,7 +93,7 @@
 
 		return !isKnownMapping && !hasExactMatch && !isInSameCategory
 			? key
 			: PropertyMappings.All.TryGetValue(canonicalName, out string? knownName) ? knownName : canonicalName;
 	}
 
 	private static bool IsInSameCategory(string key, string canonicalName)
@@ -144,8 +144,9 @@
 			return;
 		}
 
-		// Handle array/list types specially
-		if (firstValue is IList<object> || firstValue is object[])
+		// Handle array/list types specially. A lone list has nothing to merge with, so it keeps its own
+		// key like a scalar does: renaming is NameStandardizer's job, which FrontmatterNaming.AsIs turns off.
+		if (originalKeys.Count > 1 && (firstValue is IList<object> || firstValue is object[]))
 		{
 			MergeArrayValues(source, target, canonicalKey, originalKeys);
 		}
@@ -187,7 +188,7 @@
 	/// <param name="key">The key to analyze.</param>
 	/// <param name="existingKeys">All existing keys in the frontmatter.</param>
 	/// <returns>The canonical name for the key.</returns>
 	private static string FindBasicCanonicalName(string key, string[] existingKeys)
 	{
 		// First check if it's a known property
 		if (PropertyMappings.All.TryGetValue(key, out string? canonicalName))