fix: scope history record lookup by context key

SqliteDropCreateDatabaseWhenModelChanges stores the context name on each
history record and documents that a database may be shared by several
contexts, but GetHistoryRecord selected with SingleOrDefault and no Context
filter. On a shared history table that matches multiple records: IsSameModel
swallows the resulting exception and reports a model change, while SaveHistory
throws. Look the record up by context.GetType().FullName, matching what
SaveHistory writes.

Author: msallin

SQLite.CodeFirst.Test/UnitTests/Utility/HistoryRecordSelectorTest.cs

@@ -0,0 +1,56 @@
+using System;
+using System.Collections.Generic;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using SQLite.CodeFirst.Utility;
+
+namespace SQLite.CodeFirst.Test.UnitTests.Utility
+{
+    [TestClass]
+    public class HistoryRecordSelectorTest
+    {
+        private static IHistory Record(string context)
+        {
+            return new History { Context = context, Hash = "hash-" + context };
+        }
+
+        [TestMethod]
+        public void SelectForContext_ReturnsRecordForGivenContext()
+        {
+            var records = new List<IHistory> { Record("ContextA"), Record("ContextB") };
+
+            IHistory result = HistoryRecordSelector.SelectForContext(records, "ContextB");
+
+            Assert.IsNotNull(result);
+            Assert.AreEqual("ContextB", result.Context);
+        }
+
+        [TestMethod]
+        public void SelectForContext_ReturnsNull_WhenNoRecordMatches()
+        {
+            var records = new List<IHistory> { Record("ContextA") };
+
+            IHistory result = HistoryRecordSelector.SelectForContext(records, "ContextB");
+
+            Assert.IsNull(result);
+        }
+
+        [TestMethod]
+        public void SelectForContext_ReturnsNull_WhenEmpty()
+        {
+            IHistory result = HistoryRecordSelector.SelectForContext(new List<IHistory>(), "ContextA");
+
+            Assert.IsNull(result);
+        }
+
+        [TestMethod]
+        public void SelectForContext_Throws_WhenMultipleRecordsShareContext()
+        {
+            // One record per context is an invariant maintained by SaveHistory. If it is ever
+            // violated, surfacing it is better than silently picking an arbitrary record.
+            var records = new List<IHistory> { Record("ContextA"), Record("ContextA") };
+
+            Assert.ThrowsExactly<InvalidOperationException>(
+                () => HistoryRecordSelector.SelectForContext(records, "ContextA"));
+        }
+    }
+}

SQLite.CodeFirst/Internal/Utility/HistoryRecordSelector.cs

@@ -0,0 +1,19 @@
+using System.Collections.Generic;
+using System.Linq;
+
+namespace SQLite.CodeFirst.Utility
+{
+    /// <summary>
+    /// Selects the history record that belongs to a specific context from the history table.
+    /// A database can be shared by multiple contexts, so the lookup must be scoped by the context
+    /// key. Selecting without that scope would return more than one record on a shared database and
+    /// make the underlying <see cref="Enumerable.SingleOrDefault{TSource}(IEnumerable{TSource})"/> throw.
+    /// </summary>
+    internal static class HistoryRecordSelector
+    {
+        public static IHistory SelectForContext(IEnumerable<IHistory> records, string contextKey)
+        {
+            return records.SingleOrDefault(record => record.Context == contextKey);
+        }
+    }
+}

SQLite.CodeFirst/Public/DbInitializers/SqliteDropCreateDatabaseWhenModelChanges.cs

@@ -175,7 +175,11 @@ private IHistory GetHistoryRecord(TContext context)
             // in order to be supported by .NET 4.0.
             DbQuery dbQuery = context.Set(historyEntityType).AsNoTracking();
             IEnumerable<IHistory> records = Enumerable.Cast<IHistory>(dbQuery);
-            return records.SingleOrDefault();
+
+            // A database can be shared by several contexts, so the record must be looked up by the
+            // context key that SaveHistory writes. An unscoped lookup would match every context's
+            // record and throw on a shared history table.
+            return HistoryRecordSelector.SelectForContext(records, context.GetType().FullName);
         }
 
         private string GetHashFromModel(DbConnection connection)