public class Category : HasIdBase<int> { public static readonly Expression<Func<Category, bool>> NiceRating = x => x.Rating > 50; //... } var niceCategories = db.Query<Category>.Where(Category.NiceRating);
残念ながら、NiceRatingは
Expression<Func<Category, bool>>
であり、Productの場合は
Expression<Func<Product, bool>>
が必要なため、対応するカテゴリから製品を選択する場合、この数値は機能しません。 つまり、変換
Expression<Func<Category, bool>> => Expression<Func<Product, bool>>
を実行する必要があり
Expression<Func<Category, bool>> => Expression<Func<Product, bool>>
。
public class Product: HasIdBase<int> { public virtual Category Category { get; set; } //... } var niceProductsCompilationError = db.Query<Product>.Where(Category.NiceRating); // !
幸いなことに、これは非常に簡単です！
// , // , public static Expression<Func<TIn, TOut>> Compose<TIn, TInOut, TOut>( this Expression<Func<TIn, TInOut>> input, Expression<Func<TInOut, TOut>> inOutOut) { // x => blah-blah. null var param = Expression.Parameter(typeof(TIn), null); // , var invoke = Expression.Invoke(input, param); // " " var res = Expression.Invoke(inOutOut, invoke); // return Expression.Lambda<Func<TIn, TOut>>(res, param); } // "" Where public static IQueryable<T> Where<T, TParam>(this IQueryable<T> queryable, Expression<Func<T, TParam>> prop, Expression<Func<TParam, bool>> where) { return queryable.Where(prop.Compose(where)); } // [Fact] public void AdvancedWhere_Works() { var product = new Product(new Category() {Rating = 700}, "Some Product", 100500); var q = new[] {product}.AsQueryable(); var values = q.Where(x => x.Category, Category.NiceRating).ToArray(); Assert.Equal(700, values[0].Category.Rating); }
これが、 LinqKitで式の構成が実装される方法です。 ただし、Entity Frameworkは
InvokeExpression
おらず、
NotSupportedException
をスローします。 LINQがリークしていることをご存知ですか？ この制限を回避するために、LinqKitは
AsExpandable
使用します。 この問題は、Pete Montgomery のブログで説明されています。 そのバージョンのPredicate Builderはストリートマジックをあまり必要とせず、
IEnumerable<T>
と
IQueryable<T>
両方で機能します。
コードをそのまま持ってきます。
public static class PredicateBuilder { /// <summary> /// Creates a predicate that evaluates to true. /// </summary> public static Expression<Func<T, bool>> True<T>() { return param => true; } /// <summary> /// Creates a predicate that evaluates to false. /// </summary> public static Expression<Func<T, bool>> False<T>() { return param => false; } /// <summary> /// Creates a predicate expression from the specified lambda expression. /// </summary> public static Expression<Func<T, bool>> Create<T>(Expression<Func<T, bool>> predicate) { return predicate; } /// <summary> /// Combines the first predicate with the second using the logical "and". /// </summary> public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second) { return first.Compose(second, Expression.AndAlso); } /// <summary> /// Combines the first predicate with the second using the logical "or". /// </summary> public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second) { return first.Compose(second, Expression.OrElse); } /// <summary> /// Negates the predicate. /// </summary> public static Expression<Func<T, bool>> Not<T>(this Expression<Func<T, bool>> expression) { var negated = Expression.Not(expression.Body); return Expression.Lambda<Func<T, bool>>(negated, expression.Parameters); } /// <summary> /// Combines the first expression with the second using the specified merge function. /// </summary> static Expression<T> Compose<T>(this Expression<T> first, Expression<T> second, Func<Expression, Expression, Expression> merge) { // zip parameters (map from parameters of second to parameters of first) var map = first.Parameters .Select((f, i) => new { f, s = second.Parameters[i] }) .ToDictionary(p => ps, p => pf); // replace parameters in the second lambda expression with the parameters in the first var secondBody = ParameterRebinder.ReplaceParameters(map, second.Body); // create a merged lambda expression with parameters from the first expression return Expression.Lambda<T>(merge(first.Body, secondBody), first.Parameters); } class ParameterRebinder : ExpressionVisitor { readonly Dictionary<ParameterExpression, ParameterExpression> map; ParameterRebinder(Dictionary<ParameterExpression, ParameterExpression> map) { this.map = map ?? new Dictionary<ParameterExpression, ParameterExpression>(); } public static Expression ReplaceParameters(Dictionary<ParameterExpression, ParameterExpression> map, Expression exp) { return new ParameterRebinder(map).Visit(exp); } protected override Expression VisitParameter(ParameterExpression p) { ParameterExpression replacement; if (map.TryGetValue(p, out replacement)) { p = replacement; } return base.VisitParameter(p); } } }