Is it possible to retrieve an entity from gae datastore using a property and not using the key?
I could see I can retrieve entities with key using the below syntax.
quote = mgr.getObjectById(Students.class, id);
Is there an alternative that enables us to use a property instead of key?
Or please suggest any other ways to achieve the requirement.
Thanks,
Karthick.
Of course this is possible. Think of the key of an entity being like the primary key of an SQL row (but please, don't stretch the analogy too far - the point is it's a primary key - the implementations of these two data storage systems are very different and it causes people trouble when they don't keep this in mind).
You should look either here (JDO) to read about JDO queries or here (JPA) to read about JPA queries, depending what kind of mgr your post refers to. For JDO, you would do something like this:
// begin building a new query on the Cat-kind entities (given a properly annotated
// entity model class "Cat" somewhere in your code)
Query q = pm.newQuery(Cat.class);
// set filter on species property to == param
q.setFilter("species == speciesParam");
// set ordering for query results by age property descending
q.setOrdering("age desc");
// declare the parameters for this query (format is "<Type> <name>")
// as referenced above in filter statement
q.declareParameters("String speciesParam");
// run the query
List<Cat> results = (List<Cat>) q.execute ("siamese");
For JPA, you would use JPQL strings to run your queries.
Related
Consider Below is my sample json.
{
"servletname": "cofaxEmail",
"servlet-class": "org.cofax.cds.EmailServlet",
"init-param": {
"mailHost": "mail1",
"mailHostOverride": "mail2"
}
i have chosen "servletname" as my primary key as i will receive it in every request plus few 1000 server names are there it could be the best PK.
My Question is, to make the partition key work for me.
Do i have to specify the partition key option seperately like below
ItemResponse<ServerDto> ServerDtoResponse = await this.container.ReadItemAsync<ServerDto>(bocServerDto.mailHost, new PartitionKey(bocServerDto.servletname));
or
Including the partition key in the select query itself , without adding seperate new PartitionKey(), like
select * from r where r.servletname='cofaxEmail' and r.mailHost='mail1';
Crux of the question is: By passing partitionKey object in where condition of select query is it enough to utilize the partition key feature?
Thanks
For any crud operation you would pass in the value for the partition key. For example, on a point read.
ItemResponse<ServerDto> ServerDtoResponse = await this.container.ReadItemAsync<ServerDto>(bocServerDto.mailHost, new PartitionKey("cofaxEmail"));
For a query, you can either pass it in the queryRequest options or use it in the query as the first filter predicate. Here is an example of using the queryRequest options.
thanks.
First of all, I have table structure like this,
Users:{
UserId
Name
Email
SubTable1:[{
Column-111
Column-112
},
{
Column-121
Column-122
}]
SubTable2:[{
Column-211
Column-212
},
{
Column-221
Column-222
}]
}
As I am new to DynamoDB, so I have couple of questions regarding this as follows:
1. Can I create structure like this?
2. Can we set primary key for subtables?
3. Luckily, I found DynamoDB helper class to do some operations into my DB.
https://www.gopiportal.in/2018/12/aws-dynamodb-helper-class-c-and-net-core.html
But, don't know how to fetch only perticular subtable
4. Can we fetch only specific columns from my main table? Also need suggestion for subtables
Note: I am using .net core c# language to communicate with DynamoDB.
Can I create structure like this?
Yes
Can we set primary key for subtables?
No, hash key can be set on top level scalar attributes only (String, Number etc.)
Luckily, I found DynamoDB helper class to do some operations into my DB.
https://www.gopiportal.in/2018/12/aws-dynamodb-helper-class-c-and-net-core.html
But, don't know how to fetch only perticular subtable
When you say subtables, I assume that you are referring to Array datatype in the above sample table. In order to fetch the data from DynamoDB table, you need hash key to use Query API. If you don't have hash key, you can use Scan API which scans the entire table. The Scan API is a costly operation.
GSI (Global Secondary Index) can be created to avoid scan operation. However, it can be created on scalar attributes only. GSI can't be created on Array attribute.
Other option is to redesign the table accordingly to match your Query Access Pattern.
Can we fetch only specific columns from my main table? Also need suggestion for subtables
Yes, you can fetch specific columns using ProjectionExpression. This way you get only the required attributes in the result set
I have taken over some code and I see that database updates are performed like this:
dbcon = DependencyService.Get<ISQLite>().GetConnection();
public void UpdateAnswered(string id)
{
lock(locker)
{
dbcon.Query<Phrase>("UPDATE Phrase SET Answered = Answered + 1 " +
"WHERE Id = ?", id);
}
}
I am new to using SQLite with Xamarin but it looks strange to me that this update is handled with a dbcon.Query and that the table name is passed as . Can someone confirm is this the optimal way to handle a table update? Also why is it coded as a query with the table name being passed?
Update<T>
This method allows you to pass in an instance of an object that this stored in the database which has a primary key. SQLite then recognizes the primary key and updates the rest of the object's values.
You would just call connection.Update( phrase ); where the phrase is an instance of the Phrase class with properties you want to set. Be aware that all columns except ID will be updated.
Query<T>
Performs a query and returns the results. The type parameter specifies the type of the items returned. This is most appropriate for SELECT queries.
Execute
This returns the number of affected rows by the query as an int. This is probably the best choice for your UPDATE query after the Update<T> method.
ExecuteScalar<T>
Use for queries that return scalar types - like COUNT, etc., where T is the type of the value.
In summary, Update is the most natural way to update a row in the database (with an instance you have), but Query<T> and Execute<T> are very useful if you just want to UPDATE one column like in your example.
I have an Entity Match with two columns gamesP1, gamesP2. How do I query it to return all Entities where gamesP1 > gamesP2?
Datastore query filters must be written in terms of a value provided by the application at the time of the query; they can't refer to the values of other properties.
Depending on your data model, you could define another property, gamesDiff that you set to gamesP1 - gamesP2 whenever you update an entity. Then you can write the query as:
SELECT * FROM Match WHERE gamesDiff > 0
If you're using Python, check out ndb's ComputedProperty which makes this easy.
I have map the entities in .hmb.xml and define attribute for all entity in classes.
I have some basic accomplishment and get all the record using below code.
public List<DevelopmentStep> getDevelopmentSteps()
{
List<DevelopmentStep> developmentStep;
developmentStep = Repository.FindAll<DevelopmentStep>(new OrderBy("Id", Order.Asc));
return developmentStep;
}
I have check from net that we can write HQL, Now the problem is how to execute this HQL like..
string hql = "From DevelopmentSteps d inner join table2 t2 d.id=t2.Id where d.id=IDValue";
What additional Classes or other thing I need to add to execute this kind of HQL?
Please help me ---- Thanks
To write dynamic queries, I recommend using the Criteria API. This is dynamic, because you have a single query for several different types and you also want to set the ordering dynamically.
The queries are always object oriented. You don't need to join by foreign keys, you just navigate through the class model. There also no "tables" in the queries, but entities.
Getting (single) instances by ID should always be done using session.Get (or session.Load). Only then NHibernate can also take it directly from the cache without database roundtrip, it it had already been loaded.
for instance:
public IList<T> GetAll<T>(string orderBy)
{
return session.CreateCriteria(typeof(T))
.AddOrder(Order.Asc(orderBy))
.List<T>();
}