SQL Lite Xamarin : Query - sqlite

I'm newbie in SQLite.
I would like to query my SQLite database to get multiple rows.
When I add a new item in my local database I call this method Add:
public bool Add<T>(string key, T value)
{
return this.Insert(new SQliteCacheTable(key, this.GetBytes(value))) == 1;
}
_simpleCache.Add("favorite_1", data1);
_simpleCache.Add("favorite_2", data2);
_simpleCache.Add("favorite_3", data2);
Then,
I would like to retrieve from local database all entries where key starts with "favorite_"
to returns all objects in the database which are "favorite" objects.
I'm experienced in Linq, and I would like to do something like this:
IEnumerable<Element> = repository.Find((element) => element.Key.StartWith("favorite_"))
In the SQLiteConnection class there is a method like this:
SQLite.Net.SQLiteConnection.Find<T>(System.Linq.Expressions.Expression<System.Func<T,bool>>)
But I would like the same with in returns a collection IEnumerable<T>.
Can you help me please?
Thank you.
Jool

You have to build your query on the table itself, not the connection:
Assuming:
SQLiteConnection repository;
Then the code would look like:
var favorites = repository.Table<SQliteCacheTable>().Where(item => item.StartsWith("favorite_"));
The favorites variable is of type TableQuery<SQliteCacheTable> though, so it does not yet contain your data. The execution of the actual SQL query will be deferred until you try to access the results (by enumerating with foreach or converting to a list with ToList, for example).
To actually observe what's going on on the database, you can turn on tracing in sqlite-net, by setting repository.Trace = true on your SQLiteConnection object.
Finally, it's worth mentioning that you can also use the C# query syntax on TableQuery<T> objects, if your comfortable with it. So your query could become:
var favorites = from item in repository.Table<SQliteCacheTable>()
where item.StartsWith("favorite_")
select item;

Related

Handling reads of Cosmos DB container with multiple types?

I'd like to store several different object types in a single Cosmos DB container, as they are all logically grouped and make sense to read together by timestamp to avoid extra HTTP calls.
However, the Cosmos DB client API doesn't seem to provide an easy way of doing the reads with multiple types. The best solution I've found so far is to write your own CosmosSerializer and JsonConverter, but that feels clunky: https://thomaslevesque.com/2019/10/15/handling-type-hierarchies-in-cosmos-db-part-2/
Is there a more graceful way to read items of different types to a shared base class so I can cast them later, or do I have to take the hit?
Thanks!
The way I do this is to create the ItemQueryIterator and FeedResponse objects as dynamic and initially read them untyped so I can inspect a "type" property that tells me what type of object to deserialize into.
In this example I have a single container that contains both my customer data as well as all their sales orders. The code looks like this.
string sql = "SELECT * from c WHERE c.customerId = #customerId";
FeedIterator<dynamic> resultSet = container.GetItemQueryIterator<dynamic>(
new QueryDefinition(sql)
.WithParameter("#customerId", customerId),
requestOptions: new QueryRequestOptions
{
PartitionKey = new PartitionKey(customerId)
});
CustomerV4 customer = new CustomerV4();
List<SalesOrder> orders = new List<SalesOrder>();
while (resultSet.HasMoreResults)
{
//dynamic response. Deserialize into POCO's based upon "type" property
FeedResponse<dynamic> response = await resultSet.ReadNextAsync();
foreach (var item in response)
{
if (item.type == "customer")
{
customer = JsonConvert.DeserializeObject<CustomerV4>(item.ToString());
}
else if (item.type == "salesOrder")
{
orders.Add(JsonConvert.DeserializeObject<SalesOrder>(item.ToString()));
}
}
}
Update:
You do not have to use dynamic types if want to create a "base document" class and then derive from that. Deserialize into the documentBase class, then check the type property check which class to deserialize the payload into.
You can also extend this pattern when you evolve your data models over time with a docVersion property.

What do I need to learn for accessing data from mssql database?

I am learning asp.net core mvc and API. I can simply work on it for CRUD operation. But, I get confused for accessing data from multiple tables like listing all categories with showing number of items each categories contains. What I need to learn for example Lina, entity framework code first, ado.net? I am currently using entity framework code first.
Thanks
Learn Dapper a simple object mapper for .NET.
What I Understood from your problem is that you have multiple tables and you want to query them and map the query result in your c# or vb code.Here I will show simple mapping of query result to c# objects.
Suppose you have three tables category_1,category_2 and category_3.
Lets say each table have two columns named itemName and ItemValue.
Lets create c# class corresponding to each table.
class category_1{
string ItemName {get;set};
int ItemValue {get;set;};
}
class category_2{
string ItemName {get;set};
int ItemValue {get;set;};
}
class category_3{
string ItemName {get;set};
int ItemValue {get;set;};
}
Suppose your querying three tables and map the result of query to respective object in c#.
let our sql query be as follows:
string sql = #"select * from category_1;select * from category_2;select * from category_3;"
Here we have three select statements and each select statement will give you result of respective table.Lets query the above sql to database using dapper and map them to c# object as follows.
List<category_1> lstCotegory1 = new List<category_1>();
List<category_2> lstCotegory2 = new List<category_2>();
List<category_3> lstCotegory3 = new List<category_3>();
using (var multi = connection.QueryMultiple(sql))
{
lstCotegory1 = multi.Read<category_1>().ToList(); // map first select statement
lstCotegory2 = multi.Read<category_2>().ToList(); // map second select statement
lstCotegory3 = multi.Read<category_3>().ToList(); // map third select statement.
}
This is how you can return results of multiple queries and map them to appropriate object. I know you can do better than this but to understand we have to go with simple example.Hope this will help.

NHibernate: adding calculated field to query results

I have inherited an ASP.NET website built on NHibernate, with which I have no experience. I need to add a calculated field based on a column in a related table to an existing query. In SQL, this would be done easily enough using a correlated subquery:
select
field1,
field2,
(select count(field3) from table2 where table2.table1ID = table1.ID) calc_field
from
table1
where
[criteria...]
Unfortunately, of course, I can't use SQL for this. So in reality, I have three related questions:
What is the best way to trace through the web of interfaces, base classes, etc used by NHibernate in order to pinpoint the object where I need to add the field?
Having located that object, what, if anything, has to be done besides adding a public property to the object corresponding to the new field?
Are there any NHibernate-specific considerations with regard to referencing a related object in a query?
Here is the existing code that performs the search:
public INHibernateQueryable<C> Search(ISearchQuery query, string sortField)
{
_session = GetSession();
var c = _session.Linq<C>();
c.Expand("IP");
c.Expand("LL");
c.Expand("LL.Address");
c.Expand("LL.Address.City");
c.Expand("LL.Address.City.State");
c.Expand("LL.Address.City.County");
c.Expand("CE");
c.Expand("IC");
c.Expand("AR");
c.Expand("ER");
c.Expand("Status");
var res = _SearchFilters
.Where(x => x.ShouldApply(query))
.Aggregate(c, (candidates, filter) => (INHibernateQueryable<C>) filter.Filter(candidates, query));
res = SortSearch(res, sortField);
return res;
}
I appreciate any advice from experienced Hibernators.
Thanks,
Mike
If you are only interested in returning a query containing a computed value, you can still call a stored procedure in NHibernate and map the results to a POCO in the same way as you map a table for CRUD operations; obviously read-only instead of updatable.
Have a look at the ISession.CreateSQLQuery method; I can post an example from one of my projects if you need one.

EF4 throws NotSupported exception when it (imho) shouldn't

OK, this thing just puzzles me.
I have a table, say Users, with columns UserID, Name, etc. Have an object mapped to it using CTP5. So now I want to test it, and do the following:
List<User> users = new List();
// Some init code here, making say 3 users.
using (UsersDbContext)
{
// insert users
}
So far so good, works fine.
Now I want to see if the records match, so I select the users back using the following code.
using (UsersDbContext dbc = UsersDbContext.GetDbContext())
{
List<Users> usersRead = dbc.Users.Where(x => x.ID >= users[0].ID && x.ID <= users[users.Count - 1].ID).ToList();
}
This throws and exception:
System.NotSupportedException: LINQ to
Entities does not recognize the method
'User get_Item(Int32)' method, and
this method cannot be translated into
a store expression.
EF has difficulties seeing that I'm just asking to return an int in Users[0].ID ?
If I replace a call to users[0].ID with a straight int - works fine.
I get what it's trying to do, but I thought it should be pretty easy to check if the method belongs to .NET or Sql Server ?
You are trying to access an indexer in an EF expression, which doesn't translate to an SQL query. You'll have to move the parameters outside the query like this:
int first = users[0].ID;
int last = users[users.Count - 1].ID;
List<Users> usersRead = dbc.Users.Where(x => x.ID >= first && x.ID <= last).ToList();

Returning a column from a linked table in LINQ to SQL

My problem is that I am trying to return a simple query that contains an object Story. The Story object has a UserId in the table which links to aspnet_users' UserId column. I have created a partial class for Story that adds the UserName property since it does not exist in the table itself.
The following query gets all stories; however, a pagination helper takes the query and returns only what's necessary once this is passed back to the controller.
public IQueryable<Story> FindAllStories(){
var stories = (from s in db.Stories
orderby s.DateEntered descending
select new Story
{
Title = s.Title,
StoryContent = s.StoryContent,
DateEntered = s.DateEntered,
DateUpdated = s.DateUpdated,
UserName = s.aspnet_User.UserName
}
);
return stories;
}
When the helper does a .count() on the source it bombs with the following exception:
"Explicit construction of entity type 'MyWebsite.Models.Story' in query is not allowed."
Any ideas? It's not a problem with the helper because I had this working when I simply had the UserName inside the Story table. And on a side note - any book recommendations for getting up to speed on LINQ to SQL? It's really kicking my butt. Thanks.
The problem is precisely what it tells you: you're not allowed to use new Story as the result of your query. Use an anonymous type instead (by omitting Story after new). If you still want Story, you can remap it later in LINQ to Objects:
var stories = from s in db.Stories
orderby s.DateEntered descending
select new
{
Title = s.Title,
StoryContent = s.StoryContent,
DateEntered = s.DateEntered,
DateUpdated = s.DateUpdated,
UserName = s.aspnet_User.UserName
};
stories = from s in stories.AsEnumerable() // L2O
select new Story
{
Title = s.Title,
StoryContent = s.StoryContent,
...
};
If you really need to return an IQueryable from your method and still need the Username of the user you can use DataContext.LoadOptions to eagerload your aspnet_user objects.
See this example.

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