how to convert linq query into lambda expression

Question

I have written Linq Query that i wanted to make it Lambda Expression. how to write the Lambda Expression in place of return statement Linq Query. entities tables are entities.Users,entities.Users,entities.ponds

Query Expression :

from pond in Ponds
join customerdevice in CustomerDevices on pond.Imei equals customerdevice.Imei
join user in Users on customerdevice.CustomerId equals user.CustomerId
where user.Username=="user1"
select new { temp = pond.Temp, imei = pond.Imei,timestamp=pond.Timestatmp }

Lambda Expression :

 public async Task<IHttpActionResult> GetAllData(int deviceid)
        {
            using (smartpondEntities entities = new smartpondEntities())
            {
                try
                {
                    return Ok(await entities.ponds.Where(u=>u.deviceid==deviceid).OrderByDescending(u => u.timestatmp).ToListAsync());
                }
                catch (Exception Ex)
                {
                    return BadRequest("Sorry Error Found!!!");
                }
            }
        }

i have modified my question, can u plz check

krishna mohan
– krishna mohan

2020-02-17 15:12:25 +00:00
Commented Feb 17, 2020 at 15:12 — krishna mohan
– krishna mohan, Commented Feb 17, 2020 at 15:12

garty · Accepted Answer · 2020-02-17 16:16:03Z

That's pretty much a 1-1 mapping.

entities.Ponds
    .Join(entities.CustomerDevices, pond => pond.Imei, device => device.Imei, new {pond,device});
    .Join(entities.Users, devicePond => devicePond.deviceCustomerId, user => user.CustomerId, new {devicePond.pond, devicePond.device, user})
    .Where( e => e.user.Username == "user1")
    .Select( e => new { temp = e.pond.Temp, imei = e.pond.Imei,timestamp=e.pond.Timestatmp })
    .ToListAsync();

However, I'd suggest you add some navigational properties on your entities to shorten the Linq queries and have EF generate the joins for you.

public class Ponds {
    public string Imei {get;set}
    [ForeignKey(nameof(Imei))]
    public virtual CustomerDevice Device {get;set;}
}

public class CustomerDevices {
    [Key]
    public string Imei {get;set}
    public int CustomerId {get;set;}
    public int DeviceId {get;set;}
    [ForeignKey(nameof(CustomerId))]
    public virtual User Customer {get;set;}
}

public class Users { 
    [Key]
    public int CustomerId {get;set;}
}

Shorting the query to:

this.entites.Ponds.Where ( 
   e => e.Device.Customer.Username == "user1" ||
        e.Device.DeviceId == 1
)
.Select( e => new { temp = e.Temp, imei = e.Imei,timestamp=e.Timestatmp })
.FirstOrDefaultAsync();

You should then consider using something like SQLProfiler if you're using a MS DB to view, and then optimize the EF-SQL query.

jdweng · Accepted Answer · 2020-02-17 15:49:11Z

0

You lamda should be :

var ponds = from pond in Ponds
join customerdevice in CustomerDevices on pond.Imei equals customerdevice.Imei
join user in Users on customerdevice.CustomerId equals user.CustomerId
where user.Username=="user1"
select new { temp = pond.Temp, imei = pond.Imei,timestamp=pond.Timestatmp }

Then query (I replaced deviceid with imei).

return Ok(await entities.ponds.Where(u=>u.imei==deviceid).OrderByDescending(u => u.timestatmp).ToListAsync());

answered Feb 17, 2020 at 15:49

jdweng

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