Linq-复杂查询-列表中的列表

Question

I have this class:

public class RecipeLine
{
    public List<string> PossibleNames { get; set; }

    public string Name { get; set; }

    public int Index { get; set; }
}

I have a list of multiple RecipeLine objects. For example, one of them looks like this:

Name: apple
PossibleNames: {red delicious, yellow delicious, ... }
Index = 3

I also have a table in my db which is called tblFruit and has 2 columns: name and id. the id isn't the same as the index in the class.

What I want to do is this: for the whole list of RecipeLine objects, find all the records in tblFruit whose name is in PossibleNames, and give me back the index of the class and the id in the table. So we have a list in a list (a list of RecipeLine objects who have a list of strings). How can I do this with Linq in c#?

Answer 1

I'm pretty sure there isn't going to be a LINQ statement that you can construct for this that will create a SQL query to get the data exactly how you want. Assuming tblFruit doesn't have too much data, pull down the whole table and process it in memory with something like...

var result = tblFruitList.Select((f) => new {Id = f.id, Index = recipeLineList.Where((r) => r.PossibleNames.Contains(f.name)).Select((r) => r.Index).FirstOrDefault()});

Keeping in mind that Index will be 0 if there isn't a recipeLine with the tblFruit's name in it's PossibleNames list.

A more readable method that doesn't one-line it into a nasty linq statement is...

Class ResultItem {
    int Index {get;set;}
    int Id {get;set;}
}

IEnumerable<ResultItem> GetRecipeFruitList(IEnumerable<FruitItem> tblFruitList, IEnumerable<RecipeLine> recipeLineList) {
    var result = new List<ResultItem>();
    foreach (FruitItem fruitItem in tblFruitList) {
        var match = recipeLineList.FirstOrDefault((r) => r.PossibleNames.Contains(fruitItem.Name));
        if (match != null) {
            result.Add(new ResultItem() {Index = match.Index, Id = fruitItem.Id});
        }
    }
    return result;
}

If tblFruit has a lot of data you can try and pull down only those items that have a name in the RecipeLine list's of PossibleName lists with something like...

var allNames = recipeLineList.SelectMany((r) => r.PossibleNames).Distinct();
var tblFruitList = DbContext.tblFruit.Where((f) => allNames.Contains(f.Name));

Answer 2

要获取表中Name在“ PossibleNames Name所有水果，请使用以下命令：

var query = myData.Where(x => myRecipeLines.SelectMany(y => y.PossibleNames).Contains(x.Name));

Answer 3

I don't think you can do this in a single step.

I would first create a map of the possible names to indexes:

var possibleNameToIndexMap = recipes
    .SelectMany(r =>  r.PossibleNames.Select(possibleName => new { Index = r.Index, PossbileName = possibleName }))
    .ToDictionary(x => x.PossbileName, x => x.Index);

Then, I would retrieve the matching names from the table:

var matchingNamesFromTable = TblFruits
    .Where(fruit => possibleNameToIndexMap.Keys.Contains(fruit.Name))
    .Select(fruit => fruit.Name);

Then you can use the names retrieved from the tables as keys into your original map:

var result = matchingNamesFromTable
    .Select(name => new { Name = name, Index = possibleNameToIndexMap[name]});

Not fancy, but it should be easy to read and maintain.