Handle some features that came up during integration with the app

src/NexusMods.MnemonicDB.Abstractions/IDb.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Collections;
 using System.Collections.Generic;
 using NexusMods.MnemonicDB.Abstractions.Attributes;
 using NexusMods.MnemonicDB.Abstractions.DatomIterators;
@@ -77,6 +78,11 @@ public Entities<EntityIds, TModel> GetReverse<TModel>(EntityId id, Attribute<Ent
     /// </summary>
     IEnumerable<EntityId> FindIndexed<TValue, TLowLevel>(Attribute<TValue, TLowLevel> attribute, TValue value);
 
+    /// <summary>
+    /// Finds all the entity ids that have the given attribute with the given value.
+    /// </summary>
+    IEnumerable<EntityId> FindIndexed(ReferenceAttribute attribute, EntityId value);
+
     /// <summary>
     /// Finds all the datoms have the given attribute with the given value.
     /// </summary>

src/NexusMods.MnemonicDB.Abstractions/Models/ModelExtensions.cs

@@ -0,0 +1,27 @@
+using System;
+using System.Reactive.Linq;
+using DynamicData;
+using NexusMods.MnemonicDB.Abstractions.Query;
+
+namespace NexusMods.MnemonicDB.Abstractions.Models;
+
+/// <summary>
+/// Extensions for models
+/// </summary>
+public static class ModelExtensions
+{
+    /// <summary>
+    /// An observable that emits the revisions of the model, terminates when the model is deleted
+    /// or otherwise invalid
+    /// </summary>
+    public static IObservable<TModel> Revisions<TModel>(this TModel model)
+        where TModel : IReadOnlyModel<TModel>
+    {
+        // Pull these out of the model so that we don't keep the model data lying around in memory
+        var conn = model.Db.Connection;
+        var id = model.Id;
+        return conn.ObserveDatoms(id)
+            .QueryWhenChanged(_ => TModel.Create(conn.Db, id))
+            .TakeWhile(m => m.IsValid());
+    }
+}

src/NexusMods.MnemonicDB.Abstractions/Query/ObservableDatoms.cs

@@ -2,6 +2,7 @@
 using System.Collections.Generic;
 using System.Reactive.Linq;
 using DynamicData;
+using NexusMods.MnemonicDB.Abstractions.Attributes;
 using NexusMods.MnemonicDB.Abstractions.DatomComparators;
 using NexusMods.MnemonicDB.Abstractions.DatomIterators;
 using NexusMods.MnemonicDB.Abstractions.IndexSegments;
@@ -37,6 +38,30 @@ public static IObservable<IChangeSet<Datom>> ObserveDatoms(this IConnection conn
         return conn.ObserveDatoms(SliceDescriptor.Create(id, conn.Registry));
     }
 
+    /// <summary>
+    /// Observe changes for a given attribue on a given entity id
+    /// </summary>
+    public static IObservable<IChangeSet<Datom>> ObserveDatoms(this IConnection conn, EntityId id, IAttribute attribute)
+    {
+        return conn.ObserveDatoms(SliceDescriptor.Create(id, attribute.GetDbId(conn.Registry.Id), conn.Registry));
+    }
+
+    /// <summary>
+    /// Observe changes for datoms that point to the given entity id via the given attribute
+    /// </summary>
+    public static IObservable<IChangeSet<Datom>> ObserveDatoms(this IConnection conn, ReferenceAttribute attribute, EntityId id)
+    {
+        return conn.ObserveDatoms(SliceDescriptor.Create(attribute, id, conn.Registry));
+    }
+
+    /// <summary>
+    /// Observe changes for a given attribute
+    /// </summary>
+    public static IObservable<IChangeSet<Datom>> ObserveDatoms(this IConnection conn, IAttribute attribute)
+    {
+        return conn.ObserveDatoms(SliceDescriptor.Create(attribute, conn.Registry));
+    }
+
     private static IChangeSet<Datom> Diff(SortedSet<Datom> set, IndexSegment updates, SliceDescriptor descriptor, IEqualityComparer<Datom> comparer)
     {
         List<Change<Datom>>? changes = null;

src/NexusMods.MnemonicDB.Abstractions/Query/SliceDescriptor.cs

@@ -101,6 +101,19 @@ public static SliceDescriptor Create<THighLevel, TLowLevel>(Attribute<THighLevel
         };
     }
 
+    /// <summary>
+    /// Creates a slice descriptor for the given reference attribute and entity that is being pointed to.
+    /// </summary>
+    public static SliceDescriptor Create(ReferenceAttribute attr, EntityId value, IAttributeRegistry registry)
+    {
+        return new SliceDescriptor
+        {
+            Index = IndexType.VAETCurrent,
+            From = Datom(EntityId.MinValueNoPartition, attr, value, TxId.MinValue, false, registry),
+            To = Datom(EntityId.MaxValueNoPartition, attr, value, TxId.MaxValue, false, registry)
+        };
+    }
+
     /// <summary>
     /// Creates a slice descriptor for the given reference attribute and entity that is being pointed to, this is a
     /// reverse lookup.

src/NexusMods.MnemonicDB.SourceGenerator/Template.weave

@@ -34,14 +34,12 @@ public partial class {{= model.Name}} {
     #region CRUD Methods
 
     {{each attr in model.Attributes}}
-    {{if attr.IsIndexed}}
-
+    {{if attr.IsIndexed || attr.IsReference}}
     public static IEnumerable<{{= model.Name}}.ReadOnly> FindBy{{= attr.Name}}(__ABSTRACTIONS__.IDb db, {{= attr.HighLevelType.ToDisplayString()}} value) {
         foreach (var id in db.FindIndexed({{= attr.Name}}, value)) {
             yield return new {{= model.Name}}.ReadOnly(db, id);
         }
     }
-
     {{/if}}
     {{/each}}
 
@@ -245,6 +243,11 @@ public partial class {{= model.Name}} {
            /// </summary>
            public __ABSTRACTIONS__.IDb Db { get; }
 
+           /// <summary>
+           /// Rebases the entity to the most recent version of the database
+           /// </summary>
+           public ReadOnly Rebase() => new ReadOnly(Db.Connection.Db, Id);
+
            /// <summary>
            /// Constructs a new ReadOnly model of the entity.
            /// </summary>

src/NexusMods.MnemonicDB.Storage/InMemoryBackend/Snapshot.cs

@@ -1,7 +1,6 @@
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Linq;
-using Microsoft.Win32;
 using NexusMods.MnemonicDB.Abstractions;
 using NexusMods.MnemonicDB.Abstractions.IndexSegments;
 using NexusMods.MnemonicDB.Abstractions.Query;

src/NexusMods.MnemonicDB/Connection.cs

@@ -9,7 +9,6 @@
 using Microsoft.Extensions.Logging;
 using NexusMods.MnemonicDB.Abstractions;
 using NexusMods.MnemonicDB.Abstractions.BuiltInEntities;
-using NexusMods.MnemonicDB.Abstractions.ElementComparers;
 using NexusMods.MnemonicDB.Abstractions.IndexSegments;
 using NexusMods.MnemonicDB.Abstractions.Internals;
 using NexusMods.MnemonicDB.Abstractions.Query;

src/NexusMods.MnemonicDB/Db.cs

@@ -125,6 +125,13 @@ public IEnumerable<EntityId> FindIndexed<TValue, TLowLevel>(Attribute<TValue, TL
             .Select(d => d.E);
     }
 
+    public IEnumerable<EntityId> FindIndexed(ReferenceAttribute attribute, EntityId value)
+    {
+        return Snapshot
+            .Datoms(SliceDescriptor.Create(attribute, value, _registry))
+            .Select(d => d.E);
+    }
+
     public IEnumerable<Datom> FindIndexedDatoms<TValue, TLowLevel>(Attribute<TValue, TLowLevel> attribute, TValue value)
     {
         if (!attribute.IsIndexed)

tests/NexusMods.MnemonicDB.Tests/DbTests.cs

@@ -552,9 +552,6 @@ public async Task CanGetModelRevisions()
 
         var loadoutNames = new List<string>();
 
-
-        // TODO: re-enable this once we decide on how to handle revisions
-        /*
         using var subscription = loadout.Revisions()
             .Select(l => l.Name)
             .Finally(() => loadoutNames.Add("DONE"))
@@ -578,9 +575,19 @@ public async Task CanGetModelRevisions()
         loadoutNames.Count.Should().Be(4, "All revisions should be loaded");
 
         loadoutNames.Should().BeEquivalentTo(["Test Loadout", "Update 1", "Update 2", "DONE"]);
-        */
 
+    }
 
+    [Fact]
+    public async Task CanFindByReference()
+    {
+        var loadout = await InsertExampleData();
+        foreach (var mod in loadout.Mods)
+        {
+            var found = Mod.FindByLoadout(Connection.Db, mod.LoadoutId)
+                .Select(f => f.Id);
+            found.Should().Contain(mod.Id, "we can look entities via the value if they are references");
+        }
     }
 
     [Fact]