I am working on Database First Approach in Entity Framework where I have to retrieve specific columns from the Entity.
Public IQueryable<Entity.Employees> GetEmployeeName(String FName,String LName)
{
var query = (from s in Employees
where s.firstName = FName && s.lastName = LName
select new {s.firstName, s.middleName});
return query;
}
Here return statement is throwing an error where it seems that its not matching with Employees (entity) columns. Could you please help me in sorting out this issue? Thanks in advance.
You need to use == for comparison, also you need to use dynamic type as return type since you are returning a custom anonymous type. Try this
Public IQueryable<dynamic> GetEmployeeName(String FName,String LName)
{
var query=(from s in Employees
where s.firstName==FName && s.lastName==LName
select new {s.firstName,s.middleName});
return query.AsQueryable();
}
Finally you will use it like below, keep in mind that intelisense won't work on dynamic object.
var query = GetEmployeeName("Jake", "Smith");
List<dynamic> results = query.ToList();
foreach (dynamic result in results)
{
string fristName = result.FirstName;
string lastName = result.MiddleName;
}
I am using Codeigniter and am trying to use the Active Record Class for all my database operations.
However, I did run into problems when trying to convert the last bid of the following (working) query into Active Record code.
$sql = ("SELECT ac.cou_id
FROM authorcourse ac
INNER JOIN course c
ON ac.cou_id = c.cou_id
WHERE cou_name = ? // ? = $cou_name
AND cou_number = ? // ? = $cou_number
AND cou_term = ? // ? = $cou_term
AND cou_year = ? // ? = $cou_year
AND FIND_IN_SET (ac.aut_id, ?) //? = $aut_ids_string
GROUP BY ac.cou_id
HAVING COUNT(DISTINCT ac.aut_id) = ? //? = $aut_quantity
AND COUNT(DISTINCT ac.aut_id) = (SELECT COUNT(DISTINCT ac2.aut_id)
FROM authorcourse ac2
WHERE ac2.cou_id = ac.cou_id)");
$query = $this->db->query($sql, array($cou_name, $cou_number, $cou_term, $cou_year, $aut_ids_string, $aut_quantity));
Question: How do I convert the HAVING clause into valid Active Record code?
I tried using $this->db->having(); and $this->db->distinct(); but failed to combine the functions to achieve the desired result.
My (working) code so far:
$this->db->select('ac.cou_id');
$this->db->from('authorcourse ac');
$this->db->join('course c', 'c.cou_id = ac.cou_id');
$this->db->where('cou_name', $cou_name);
$this->db->where('cou_number', $cou_number);
$this->db->where('cou_term', $cou_term);
$this->db->where('cou_year', $cou_year);
$this->db->where_in('ac.aut_id',$aut_ids);
$this->db->group_by('ac.cou_id');
// missing code
Thanks a lot!
You code seems quite clumsy. But, you can use having clause in the following way:
$this->db->having("ac.aut_id = '".$aut_qty."'", null, false)
So, I am having an issue with an SQL query that is translated LINQ (and works - tested), but that same LINQ query does not work in Lightswitch. Of course I did not expect to work straight out, but I am struggling to properly convert it.
So here is a image of the tables that I base my query on:
http://dl.dropbox.com/u/46287356/tables.PNG
(sorry for outside link, but not enough rep points :))
The SQL query is the following:
SELECT WorkingUnits.Name AS WUName, ContractPositions.WUInstanceId,
Materials.Cost, BillingValues.Value, BillingValues.PricePerUnit
FROM WorkingUnits
INNER JOIN
Materials ON WorkingUnits.Id = Materials.Material_WorkingUnit
INNER JOIN ContractPositions ON
Materials.Id = ContractPositions.ContractPosition_Material
INNER JOIN BillingValues ON
ContractPositions.Id = BillingValues.BillingValue_ContractPosition
Now, I have transformed this to LINQ in the following way:
var query = from wu in this.DataWorkspace.ApplicationData.WorkingUnits
join m in this.DataWorkspace.ApplicationData.Materials on
new { Id = WorkingUnits.Id } equals new { Id = m.Material_WorkingUnit }
join cp in this.DataWorkspace.ApplicationData.ContractPositions on
new { Id = m.Id } equals new { Id = cp.ContractPosition_Material }
join bv in this.DataWorkspace.ApplicationData.BillingValues on
new { Id = cp.Id } equals new { Id = bv.BillingValue_ContractPosition }
select new
{
usage = bv.Value * bv.PricePerUnit,
totalCost = (bv.Value * bv.PricePerUnit) * m.Cost,
amount = (bv.Value*bv.PricePerUnit) * m.Cost / wu.WUPrice
};
Notice that I have changed a few things - like section of colums, as I do not need that in Lightswitch.
So while this works agains the SQL server, Lightswitch complains that I must consider explicitly specifying the type of the range variable 'WorkingUnits'.
I tried to cast it, but then there are other errors such as:
'int' does not contain a definition for 'Id' and no extension method 'Id'
accepting a first argument of type 'int' could be found (are you missing
a using directive or an assembly reference?)
So my questions is, how do I properly convert that query and expect it to work?
Also, If we take that my database is setup correctly, do I even need to use 'joins' in the LINQ?
Any ideas are appreciated!
try something like this
var query = from wu in this.DataWorkspace.ApplicationData.WorkingUnits
join m in this.DataWorkspace.ApplicationData.Materials on
wu.Id equals m.WorkingUnitID }
join cp in this.DataWorkspace.ApplicationData.ContractPositions on
m.Id equals cp.ContractPosition_Material
join bv in this.DataWorkspace.ApplicationData.BillingValues on
cp.Id equals bv.BillingValue_ContractPosition
select new
{
usage = bv.Value * bv.PricePerUnit,
totalCost = (bv.Value * bv.PricePerUnit) * m.Cost,
amount = (bv.Value*bv.PricePerUnit) * m.Cost / wu.WUPrice
};
What about starting at the bottom (BillingValues) and working your way up using the entity references?
eg
var query = from bv in this.DataWorkspace.ApplicationData.BillingValues
let m = bv.ContractPosition.Material
let wu = m.WorkingUnit
select new
{
usage = bv.Value * bv.PricePerUnit,
totalCost = (bv.Value * bv.PricePerUnit) * m.Cost,
amount = (bv.Value*bv.PricePerUnit) * m.Cost / wu.WUPrice
};
I'm currently using Entity Framework for my db access but want to have a look at Dapper. I have classes like this:
public class Course{
public string Title{get;set;}
public IList<Location> Locations {get;set;}
...
}
public class Location{
public string Name {get;set;}
...
}
So one course can be taught at several locations. Entity Framework does the mapping for me so my Course object is populated with a list of locations. How would I go about this with Dapper, is it even possible or do I have to do it in several query steps?
Alternatively, you can use one query with a lookup:
var lookup = new Dictionary<int, Course>();
conn.Query<Course, Location, Course>(#"
SELECT c.*, l.*
FROM Course c
INNER JOIN Location l ON c.LocationId = l.Id
", (c, l) => {
Course course;
if (!lookup.TryGetValue(c.Id, out course))
lookup.Add(c.Id, course = c);
if (course.Locations == null)
course.Locations = new List<Location>();
course.Locations.Add(l); /* Add locations to course */
return course;
}).AsQueryable();
var resultList = lookup.Values;
See here https://www.tritac.com/blog/dappernet-by-example/
Dapper is not a full blown ORM it does not handle magic generation of queries and such.
For your particular example the following would probably work:
Grab the courses:
var courses = cnn.Query<Course>("select * from Courses where Category = 1 Order by CreationDate");
Grab the relevant mapping:
var mappings = cnn.Query<CourseLocation>(
"select * from CourseLocations where CourseId in #Ids",
new {Ids = courses.Select(c => c.Id).Distinct()});
Grab the relevant locations
var locations = cnn.Query<Location>(
"select * from Locations where Id in #Ids",
new {Ids = mappings.Select(m => m.LocationId).Distinct()}
);
Map it all up
Leaving this to the reader, you create a few maps and iterate through your courses populating with the locations.
Caveat the in trick will work if you have less than 2100 lookups (Sql Server), if you have more you probably want to amend the query to select * from CourseLocations where CourseId in (select Id from Courses ... ) if that is the case you may as well yank all the results in one go using QueryMultiple
No need for lookup Dictionary
var coursesWithLocations =
conn.Query<Course, Location, Course>(#"
SELECT c.*, l.*
FROM Course c
INNER JOIN Location l ON c.LocationId = l.Id
", (course, location) => {
course.Locations = course.Locations ?? new List<Location>();
course.Locations.Add(location);
return course;
}).AsQueryable();
I know I'm really late to this, but there is another option. You can use QueryMultiple here. Something like this:
var results = cnn.QueryMultiple(#"
SELECT *
FROM Courses
WHERE Category = 1
ORDER BY CreationDate
;
SELECT A.*
,B.CourseId
FROM Locations A
INNER JOIN CourseLocations B
ON A.LocationId = B.LocationId
INNER JOIN Course C
ON B.CourseId = B.CourseId
AND C.Category = 1
");
var courses = results.Read<Course>();
var locations = results.Read<Location>(); //(Location will have that extra CourseId on it for the next part)
foreach (var course in courses) {
course.Locations = locations.Where(a => a.CourseId == course.CourseId).ToList();
}
Sorry to be late to the party (like always). For me, it's easier to use a Dictionary, like Jeroen K did, in terms of performance and readability. Also, to avoid header multiplication across locations, I use Distinct() to remove potential dups:
string query = #"SELECT c.*, l.*
FROM Course c
INNER JOIN Location l ON c.LocationId = l.Id";
using (SqlConnection conn = DB.getConnection())
{
conn.Open();
var courseDictionary = new Dictionary<Guid, Course>();
var list = conn.Query<Course, Location, Course>(
query,
(course, location) =>
{
if (!courseDictionary.TryGetValue(course.Id, out Course courseEntry))
{
courseEntry = course;
courseEntry.Locations = courseEntry.Locations ?? new List<Location>();
courseDictionary.Add(courseEntry.Id, courseEntry);
}
courseEntry.Locations.Add(location);
return courseEntry;
},
splitOn: "Id")
.Distinct()
.ToList();
return list;
}
Something is missing. If you do not specify each field from Locations in the SQL query, the object Location cannot be filled. Take a look:
var lookup = new Dictionary<int, Course>()
conn.Query<Course, Location, Course>(#"
SELECT c.*, l.Name, l.otherField, l.secondField
FROM Course c
INNER JOIN Location l ON c.LocationId = l.Id
", (c, l) => {
Course course;
if (!lookup.TryGetValue(c.Id, out course)) {
lookup.Add(c.Id, course = c);
}
if (course.Locations == null)
course.Locations = new List<Location>();
course.Locations.Add(a);
return course;
},
).AsQueryable();
var resultList = lookup.Values;
Using l.* in the query, I had the list of locations but without data.
Not sure if anybody needs it, but I have dynamic version of it without Model for quick & flexible coding.
var lookup = new Dictionary<int, dynamic>();
conn.Query<dynamic, dynamic, dynamic>(#"
SELECT A.*, B.*
FROM Client A
INNER JOIN Instance B ON A.ClientID = B.ClientID
", (A, B) => {
// If dict has no key, allocate new obj
// with another level of array
if (!lookup.ContainsKey(A.ClientID)) {
lookup[A.ClientID] = new {
ClientID = A.ClientID,
ClientName = A.Name,
Instances = new List<dynamic>()
};
}
// Add each instance
lookup[A.ClientID].Instances.Add(new {
InstanceName = B.Name,
BaseURL = B.BaseURL,
WebAppPath = B.WebAppPath
});
return lookup[A.ClientID];
}, splitOn: "ClientID,InstanceID").AsQueryable();
var resultList = lookup.Values;
return resultList;
There is another approach using the JSON result. Even though the accepted answer and others are well explained, I just thought about an another approach to get the result.
Create a stored procedure or a select qry to return the result in json format. then Deserialize the the result object to required class format. please go through the sample code.
using (var db = connection.OpenConnection())
{
var results = await db.QueryAsync("your_sp_name",..);
var result = results.FirstOrDefault();
string Json = result?.your_result_json_row;
if (!string.IsNullOrEmpty(Json))
{
List<Course> Courses= JsonConvert.DeserializeObject<List<Course>>(Json);
}
//map to your custom class and dto then return the result
}
This is an another thought process. Please review the same.
I am new to android programming, I am doing a simple SELECT with a rawquery and it is giving me an error...
Here's my code
public Cursor getSubCategory(int categoryID){
String select = "SELECT subcategory_name FROM subcategory WHERE id_category = " + categoryID;
return mDb.rawQuery(select, null);
}
As you can see the id_category is an Integer
If anyone has ideas it would be great
Your not using the API to its full advantage there you should use
String select = "SELECT subcategory_name FROM subcategory WHERE id_category = ?"
and then pass in the categoryID to the second argument like
...
String[] arguments = { categoryID.toString() }
return mDb.rawQuery(select, arguments);
...
This should remove SQL injection risks as you are using parameters (the "?").
Apart from that we will need more details about the error to help you further