Use Include() method in repository

问题:

I have the following with EF 5:

var a = context.Posts.Include(x => x.Pack).Select(x => x.Pack.Id).ToList();

This works. Then I tried to replicate this in my generic repository:

public IQueryable<T> Include<T>(Expression<Func<T, Boolean>> criteria) where T : class
{
    return _context.Set<T>().Include(criteria);
}

But in this case I am not able to do the following:

var b = repository.Include<Post>(x => x.Pack).Select(x => x.Pack.Id).ToList();

I get the error:

Cannot implicitly convert type 'Data.Entities.Pack' to 'bool'

How can I solve this?

What should I change in my Include() method?

回答1:

Try:

Change

Expression<Func<T, Boolean>> criteria

To

Expression<Func<T, object>> criteria

Edit: To Include multiple entities, you need to add an "include" extension:

public static class IncludeExtension
{
    public static IQueryable<TEntity> Include<TEntity>(this IDbSet<TEntity> dbSet,
                                            params Expression<Func<TEntity, object>>[] includes)
                                            where TEntity : class
    {
        IQueryable<TEntity> query = null;
        foreach (var include in includes)
        {
            query = dbSet.Include(include);
        }

        return query == null ? dbSet : query;
    }
}

Then you can use it like this:

repository.Include(x => x.Pack, x => x.Pack.Roles, ...).Select(x => x.Pack.Id).ToList();

Just make sure "repository" return a "DbSet" Object.

回答2:

I used the accepted answer but had to modify it a bit for EntityFramework Core. In my experience I had to keep the chain going or the previous query references were overwritten.

    public IQueryable<TEntity> Include(params Expression<Func<TEntity, object>>[] includes)
    {
        IIncludableQueryable<TEntity, object> query = null;

        if(includes.Length > 0)
        {
            query = _dbSet.Include(includes[0]);
        }
        for (int queryIndex = 1; queryIndex < includes.Length; ++queryIndex)
        {
            query = query.Include(includes[queryIndex]);
        }

        return query == null ? _dbSet : (IQueryable<TEntity>)query;
    }

回答3:

This is what we ended by doing in EF 6

 public IEnumerable<T> GetIncludes(params Expression<Func<T, Object>>[] includes)
        {
            IQueryable<T> query = table.Include(includes[0]);
            foreach (var include in includes.Skip(1))
            {
                query = query.Include(include);
            }               
            return query.ToList();
        }