在c＃EF6中返回相关集合的子集

Question

I have a webservice that allows the user to query a give subset of data from a database. I will only know which fields the user wants at runtime. It's quite easy to build a dynamic linq query to retrieve only the corresponding properties from the database for the fields in the object table itself, but how can I dynamically specify which fields I want to retrive in the related table ? Sounds confusing, let me explain with a sample.

So let's say I have a structure like:

class User {
 public string Name {get; set;}
 public List<Role> Roles {get; set;}
}

class Role {
  public string RoleName {get; set;}
  public string RoleDescription {get; set;}
}

So User and Role are the objects used by EF6 in my code first databse table generation.

Now I have webservice that wants to get the username and the name of the roles from the database. It will send a parameter like

var requestData = "{fields: {'Name', 'Roles.RoleName'}}"`;

What I would like to do is only retrieve the RoleName field from the Roles collection, knowing another client might ask the Roles.RoleDescription only so I need to dynamically create the query. Ideally I should be able to generate this at the Select level on an IQueryable object in order to avoid getting a huge number of data from the database and only using one field.

What I have tried so far:

Using System.Linq.Dynamic to retrieve the properties of the User table. Something like:

var dynamicFields = GetFieldsFromRequestData(requestData); // results in "Name, Roles" 
var query = context.Set<User>().Select(dynamicFields);
query.Load();

Works like a charm. BUT will query all fields from the Roles collection. I don't seem to be able to only select the Roles.RoleName column from the Roles table using the same strategy, which is what I want.

var query = context.Set<User>().Select("Name, Roles.RoleName");

Does not work.

I also tried using Linq.Expressions which requires specifying the objects types:

class UserDTO {
 public string Name {get; set;}
 public List<RoleDTO> Roles {get; set;}
}

class RoleDTO {
 public string RoleName {get; set;}
}

then using Linq.Expressions I generate

var generatedExpression = GetFieldsFromRequestDataLinq(requestData); //
results in an expression = (user => New UserDTO {Name = user.Name, Roles = user.Roles });
var query = context.Set<User>().Select(generatedExpression);

Works like a charm EF converts the objects in the Roles collection to RoleDTO objects.

BUT this implies knowing in advance which fields will be queried in the Roles collection since I defined the RoleDTO type. Which is not what I want since I don't know the list of queried fields.

So if someone has a nice way of solving this issue that would be awesome. I am open to AutoMapper solutions, to Newtonsoft JSON approaches, ...

Have been battling with this since 10 days ;-)

Answer 1

you should be able to add properties to anonymous type on runtime:

Add property to anonymous type after creation

Utilitizing reflection the query could look like:

    var query = context.Set<User>().Select(p=>{ 
        var dto = new UserDTO{};

        parameterNames.foreach(pn =>{
            var value = p.GetField( pn ).GetValue (p, null);
            UserDTO.Add(parameter, value);
        });
        return dto;
    });

Answer 2

Based on the context you provided I would suggest to try out OData/GraphQL .

It is usually flexible enough to be able to handle such kind of scenarios but can cause some other struggles.