## v0.9.16

- 增加 BaseRepository.AttachOnlyPrimary 方法，只附加实体的主键值；
> 在更新前使用可实现不查询数据库再更新、也可以实现更新时不更新值为 null 的字段
```csharp
class T {
    public int id { get; set; }
    public string name { get; set; }
    public string other { get; set; }
}
var item = new T { id = 1, name = "xx" };
fsql.GetRepository<T>().AttachOnlyPrimary(item).Update(item); //只更新 name
```
- 修复 Lambda 表达式中 DateTime.Now.ToString("yyyyMMdd") 不能直接执行的 bug；

FreeSql.DbContext/DbContext/DbContext.cs

@@ -129,6 +129,17 @@ namespace FreeSql
         /// <param name="data"></param>
         public void Attach<TEntity>(TEntity data) where TEntity : class => this.Set<TEntity>().Attach(data);
         public void AttachRange<TEntity>(IEnumerable<TEntity> data) where TEntity : class => this.Set<TEntity>().AttachRange(data);
+
+        /// <summary>
+        /// 附加实体，并且只附加主键值，可用于不更新属性值为null或默认值的字段
+        /// </summary>
+        /// <typeparam name="TEntity"></typeparam>
+        /// <param name="data"></param>
+        public DbContext AttachOnlyPrimary<TEntity>(TEntity data) where TEntity : class
+        {
+            this.Set<TEntity>().AttachOnlyPrimary(data);
+            return this;
+        }
         #endregion
 
         #region Queue Action

FreeSql.DbContext/DbSet/DbSet.cs

@@ -162,6 +162,23 @@ namespace FreeSql
                 });
             }
         }
+        /// <summary>
+        /// 附加实体，并且只附加主键值，可用于不更新属性值为null或默认值的字段
+        /// </summary>
+        /// <param name="data"></param>
+        public DbSet<TEntity> AttachOnlyPrimary(TEntity data)
+        {
+            if (data == null) return this;
+            var pkitem = (TEntity)Activator.CreateInstance(_entityType);
+            foreach (var pk in _db.Orm.CodeFirst.GetTableByEntity(_entityType).Primarys)
+            {
+                var colVal = _db.Orm.GetEntityValueWithPropertyName(_entityType, data, pk.CsName);
+                _db.Orm.SetEntityValueWithPropertyName(_entityType, pkitem, pk.CsName, colVal);
+            }
+            this.Attach(pkitem);
+            return this;
+        }
+
         /// <summary>
         /// 清空状态数据
         /// </summary>

FreeSql.DbContext/FreeSql.DbContext.csproj

@@ -2,7 +2,7 @@
 
     <PropertyGroup>
         <TargetFrameworks>netstandard2.0;net45</TargetFrameworks>
-        <Version>0.9.15</Version>
+        <Version>0.9.16</Version>
         <GeneratePackageOnBuild>true</GeneratePackageOnBuild>
         <Authors>YeXiangQin</Authors>
         <Description>FreeSql is the most convenient ORM in dotnet. It supports Mysql, Postgresql, SqlServer, Oracle and Sqlite.</Description>

FreeSql.DbContext/FreeSql.DbContext.xml

@@ -39,6 +39,13 @@
             <typeparam name="TEntity"></typeparam>
             <param name="data"></param>
         </member>
+        <member name="M:FreeSql.DbContext.AttachOnlyPrimary``1(``0)">
+            <summary>
+            附加实体，并且只附加主键值，可用于不更新属性值为null或默认值的字段
+            </summary>
+            <typeparam name="TEntity"></typeparam>
+            <param name="data"></param>
+        </member>
         <member name="P:FreeSql.DbContextOptions.EnableAddOrUpdateNavigateList">
             <summary>
             是否开启一对多，联级保存功能
@@ -57,6 +64,12 @@
             </summary>
             <param name="data"></param>
         </member>
+        <member name="M:FreeSql.DbSet`1.AttachOnlyPrimary(`0)">
+            <summary>
+            附加实体，并且只附加主键值，可用于不更新属性值为null或默认值的字段
+            </summary>
+            <param name="data"></param>
+        </member>
         <member name="M:FreeSql.DbSet`1.FlushState">
             <summary>
             清空状态数据
@@ -156,6 +169,12 @@
             </summary>
             <param name="entity"></param>
         </member>
+        <member name="M:FreeSql.IBasicRepository`1.AttachOnlyPrimary(`0)">
+            <summary>
+            附加实体，并且只附加主键值，可用于不更新属性值为null或默认值的字段
+            </summary>
+            <param name="data"></param>
+        </member>
         <member name="P:FreeSql.IUnitOfWork.Enable">
             <summary>
             是否启用工作单元

FreeSql.DbContext/Repository/Repository/BaseRepository.cs

@@ -140,6 +140,11 @@ namespace FreeSql
 
         public void Attach(TEntity data) => _db.Attach(data);
         public void Attach(IEnumerable<TEntity> data) => _db.AttachRange(data);
+        public IBasicRepository<TEntity> AttachOnlyPrimary(TEntity data)
+        {
+            _db.AttachOnlyPrimary(data);
+            return this;
+        }
         public void FlushState() => _dbset.FlushState();
 
         public TEntity InsertOrUpdate(TEntity entity)

FreeSql.DbContext/Repository/Repository/IBasicRepository.cs

@@ -21,6 +21,12 @@ namespace FreeSql
         /// <param name="entity"></param>
         void Attach(TEntity entity);
         void Attach(IEnumerable<TEntity> entity);
+        /// <summary>
+        /// 附加实体，并且只附加主键值，可用于不更新属性值为null或默认值的字段
+        /// </summary>
+        /// <param name="data"></param>
+        IBasicRepository<TEntity> AttachOnlyPrimary(TEntity data);
+
         int Update(TEntity entity);
         int Update(IEnumerable<TEntity> entitys);
         Task<int> UpdateAsync(TEntity entity);