Selecting Data Using Entity Framework based on optional search parameters

问题

I have a project that using EF to interface with the database. It does this via a WCF web service. This is my first EF project and I am trying to work out how to construct a query based on optional parameters.

My web service has a data contract with matching fields to the classes in EF. The web service wueries the database, populates these client classes an passes them back to the client. For an example....

In the web service

[DataContract]
public class MyMovie
{
    [DataMember]
    public int MovieId;
    [DataMember]
    public string MovieName;
    [DataMember]
    public int MovieRatingId;
    [DataMember]
    public string MovieRunTime;
    [DataMember]
    public string MovieIMDBUrl;
    [DataMember]
    public DateTime MovieDateAdded;
    [DataMember]
    public List<MyMovieActor> Actors;
    [DataMember]
    public List<MyMovieMedia> MediaFiles;
}
public class MyMovieActor
{
    [DataMember]
    public int ActorId;
    [DataMember]
    public int ActorMovieId;
    [DataMember]
    public string ActorName;
    [DataMember]
    public string ActorCharacter;
}
public class MyMovieMedia
{
    [DataMember]
    public int MediaId;
    [DataMember]
    public int MediaMovieId;
    [DataMember]
    public string MediaFileLocation;
    [DataMember]
    public bool MediaDefault;
}

The data model (simplified)...

My web service method....

    public MyMovie GetMovie(int movieId)
    {
        MyMovie response = new MyMovie();

        using (MovieDatabaseEntities DbContext = new MovieDatabaseEntities())
        {
            var query = (from oData in DbContext.Movies
                         where oData.MovieId == movieId
                         select oData).ToList();


            if (query.Count > 0)
            {
                response.MovieDateAdded = query[0].MovieDateAdded;
                response.MovieId = query[0].MovieId;
                response.MovieIMDBUrl = query[0].MovieIMDBUrl;
                response.MovieName = query[0].MovieName;
                response.MovieRatingId = query[0].MovieRatingId;
                response.MovieRunTime = query[0].MovieRunTime;
            }

            List<MyMovieActor> actors = new List<MyMovieActor>();
            foreach (MovieActor ma in query[0].MovieActors)
            {
                MyMovieActor a = new MyMovieActor();
                a.ActorId = ma.ActorId;
                a.ActorMovieId = ma.ActorMovieId;
                a.ActorCharacter = ma.ActorCharacter;
                a.ActorName = ma.ActorName;
                actors.Add(a);
            }
            response.Actors = actors;

            List<MyMovieMedia> medias = new List<MyMovieMedia>();
            foreach (MovieMedia mm in query[0].MovieMedias)
            {
                MyMovieMedia med = new MyMovieMedia();
                med.MediaId = mm.MediaId;
                med.MediaMovieId = mm.MediaMovieId;
                med.MediaFileLocation = mm.MediaFileLocation;  //HDD Folder 
                med.MediaDefault = mm.MediaDefault; //Image to show on listing
                medias.Add(med);
            }
            response.MediaFiles = medias;
        }
        return response;
    }

So now I want to create

public List<MyMovie> GetMovies(string moviename, int movierating, string movieruntime)

Where all parameters will be populated or zero length strings. If they have a values I want to add that as part of the where clause.

so basically update

                var query = (from oData in DbContext.Movies
                         where oData.MovieId == movieId
                         select oData).ToList();

to the equivalent of

            var query = (from oData in DbContext.Movies
                         where 1=1
                   if(String.IsSafe(moviename))
                   {
                         && oData.MovieName Like %movieId%
                   }
                   if(movierating != 0)
                   {
                         && oData.MovieRating = movierating
                   }
                   if(String.IsSafe(movieruntime))
                   {
                         && oData.MovieRuntime == movieruntime
                   }
                         select oData).ToList();

Looking into this has just confused me more, I have always writen my data layers using good old ADO.NET, DataReaders, Adapters, Stored Procs etc but with this project I have been asked to use EF. It is confusing me so much. What is it they say about old dogs and new tricks?!

Any help with this would be very much appreciated.

回答1:

IQueryable is your friend here. It allows for composing the query in multiple steps, but it won't generate the SQL and execute it until it's needed (like due to iteration or a ToList() call). So your code would look like:

var query = (from oData in DbContext.Movies select oData);
if (!string.IsNullOrEmpty(moviename))
  query = query.Where(m => m.MovieName.Contains(moviename));
...

来源：https://stackoverflow.com/questions/22122618/selecting-data-using-entity-framework-based-on-optional-search-parameters