Linq加入实体无法计数

Question

I have a an entity let's say call it Blog . Each Blog entity has many Keyword . The 2 are related via an fk on the keyword table.

I have a query with a signature like this FindAllBlogByKeywords(IEnumerable<string> keywords)

How would I write a linq query to pull any blog that matches ALL of the keywords in the parameter.

The problem is EF is not allowing a join between enumerable and tables. It's not contains either because I want the query to match ALL keywords, not just any one - besides I have 2 keywords list, I need the keyword in the parameter to be a subset of the keywords in the persistence.

Answer 1

You can add .Where clauses for each keyword:

IEnumerable<Blog> FindAllBlogByKeywords(IEnumerable<string> keywords)
{
     IQueryable<Blog> query = context.Blogs;

     foreach(string key in keywords)
         query = query.Where(blog => blog.Keywords.Contains(key));

     // Execute query (optional)
     return query.ToList();
}

This works because the query execution is deferred until the end. By chaining multiple .Where clauses, you effectively create an "ALL" statement.