There are two ways for DDBMapper to call batchLoad, differing in different pass-in parameters.
public Map<String,List<Object>> batchLoad(Iterable<? extends Object> itemsToGet)
public Map<String,List<Object>> batchLoad(Map<Class<?>,List<KeyPair>> itemsToGet)
I understand the second way, which makes more sense to me by specifying keyPair.
Then what about the first one? So basically just to pass in a list? Then whats the difference? The second one obviously looks more complicated
Imagine I have a User object with partition key userId and range key createdDate. I want to batch load 3 Users.
In the second option I have to create 3 key-pairs of userId and createdDate. In the first option I instantiate 3 User objects using userId and createdDate and put them in a List.
The first option might be more appropriate if I have logic in the User constructor. For example maybe createdDate cannot be more than 1 year ago. In this case creating User objects is an advantage as the constructor logic is executed. Alternatively I may have been passed the User object from some other part of the application, in which case creating key-pairs from them is just extra code I shouldn't need to write.
So basically there isn't much difference. I suspect some people will find the first option more pleasing since DynamoDBMapper is an object persistence solution, so it should support passing objects (not undefined key-pairs) around.
Related
Problem: Whenever I add an order to the orders array, an additional nested array element(-KOPWA...) gets added. I wouldn't mind except, I don't know how to access that nested string to access it's child nodes.
Example of database node for users below:
firebase.database().ref('users/'+userIdState+'/orders/'+<<unique numbervariable>>).push({
"order":{"test":"product","quantity":2}
});
I'm using the above code to push new json objects with a unique number to the firebase array. Still the nested array with the weird strings gets generated.
Can anyone help me understand how to either: create my own nested array with my own unique string or how to access the nested string that gets generated from firebase so I can access it's children nodes.
Multiple instances of nest arrays will be generated by users.
Any help is much appreciated.
Thanks,
Moe
You're experiencing this behaviour because Firebase's push is not the same as an array push. I recommend reading this article to understand how it works.
As for a solution, you can simply change push to set in your code. This will create the structure you were (presumably) expecting, that is
1:
order:
...
This is however potentially unsafe, if you allow concurrent writes (i. e. if the "unique number" in your example is not always unique).
Afaik Firebase recommends using push to safely create collections/"arrays". You can retrieve the generated key by calling the key property on the reference returned by push. Like this:
var ref = firebase.database().ref('users/'+userIdState+'/orders/'+<<unique numbervariable>>).push({
"order":{"test":"product","quantity":2}
});
var generatedKey = ref.key; // the value you're looking for
If you decide to use it, you can probably just drop the order number you have right now and just use the generated one.
Please explain above question with example scenario I am confusing which is best.
If you to fetch a specific object based on keyword or any identity in list then you have to iterate the list get object and compare with its values
In map you can directly create key value pair..you can pass key and get the value.
ex:
A object user is present which has several properties one of them is user code
Now if you have list of user object then you will fetch one by one user object and compare the code of each user...but in map you can directly store user object with user code as key pass the key and get the desired object
map.get("key");
but if you requirement is not based on key type access better to use list.. example as you to just display list of items or you have to perform sublisting.
Too broad question, but will try to shorten it:
When you have to get the value based on key (key can be anything) then you go for hashmap. Consider a telephone directory where you go to appropriate name and search for person's name to find his number.
While if you have similar object's and want to store them somehow and later on retrieve it say by index or traverse them one by one then you go for list. So if your task is to find employees older than age 50 yrs, you can just return a list of employees who are older than 50.
Using EF6 Framework4.5 – Creating my first n-tier app and first EF experience. I have CRUD working but one issue that I have a work-a-around but don’t like it. There must be a better way.
When data object is returned from my UI layer to the DAL layer, it has been detached, so I flag EntityState as “Modified.” But then it updates all columns in the db. Values that were not loaded in the form view (and not submitted) obviously are null and updated to such in the db.
1) My first solution does work:
Store the object in session in the UI layer and loop through the object updating edited values when the form is submitted. Thus, original values are passed back unchanged and updated to original values. I don’t think this would be best practice though.
2) The solution I think I want:
I am looking for a helper function in the DAL layer to loop through all values in the returned object and flag only non-null values as “IsModified” before calling SaveChanges.
I have found examples in C# on how to check for changed value but not null. (I am still a vb guy anyway. Don't hate.)
A) Is solution #2 a good way to do this?
B) Has anyone a piece of good to help me?
Thank you.
BTW, this is my best stab at it so far: (Errors on “CurrentValues”)
Public Overridable Function MarkEntriesModified(entity As Object)
Dim dbEntityEntry = DbContext.Entry(entity)
'Ensure only non-null values are inserted
For Each [property] In dbEntityEntry.CurrentValues.PropertyNames
If Not IsDBNull(dbEntityEntry.CurrentValues.GetValue(Of Object)([property])) Then
dbEntityEntry.[Property]([property]).IsModified = True
End If
Next
Return entity
Try this architecture. So if you are using EF then I suppose you have edmx updated and correctly representing your database objects.
eg: Say you wan to update Customer data in customer table
Create a Customer class.
when you extract a particular customer, get an instance of the
customer created.
pass this instance to the UI
and pass the instance back to Business layer to save
this way you don't loose anything in between.
some sample code
I have a table in my SQL Server DB that holds auditing information for certain actions a user takes within my system. Things like who performed the action, when it was performed, and what action are all pieces of information that can easily span multiple actions. But depending on the action performed, there may be other information that I want to capture, that is specific to the action. To handle this, I elected to add an "XML Metadata" column to the table that holds serialized XML of different metadata objects that I've created. I created a metadata object for each of the actions that I'm interested in tracking extra for. So each object is responsible for tracking specific extra information (metadata) for it's action. The objects are serialized and written to my new column.
I have SystemAction objects that I use to store information from this table, and I've added a string field that holds the XML string from the DB. The problem is, when I'm reading this XML back from the SystemAction objects, I'm struggling with a way to generically translate it back into it's correct metadata object. Each metadata object is going to have different fields, and each object has it's own static method that takes an XML string and attempts to return the metadata object type. So I could say:
SomeActionMetadata mdObj = SomeActionMetadata.BuildFromXML(xmlStringFromDB);
But I really don't know of a way to say "Here's some XML that could translate to any number of different objects. Figure it out and give me the right object back."
Given my current implementation, I could always just assign a unique ID to each metadata object that is stored as a field in each object, then use a case statement to switch on that ID and use the appropriate class's static build method to build the right object. But I was hoping for something a little more automatic than that. What if I have a List of SystemAction objects and just want to loop through them and generate the correct metadata object type?
I was hoping someone might have run across something similar to this before, or could point me to an article or post that could help me out. Thanks very much.
As indicated by Subhash Dike in the comments below, there is a similar SO question here that was able to point me in the right direction.
Just dipping my toes into Linq2sql project after years of rolling my own SQL Server DB access routines.
Before I spend too much time figuring out how to make linq2sql behave like my custom code used to, I want to check to make sure that it isn't already "built" in behavior that I can just use by setting up the relationships right in the designer...
Very simple example:
I have two tables: Person and Notes, with a 1 to many relationship (1 Person, many notes), linked by Person.ID->Note.PersonID.
I have a stored procedure (all data access is done via SP's and I plan on continuing that) which makes the Link2SQL a bit more work for me.
sp_PersonGet(#ID int) which returns the person record and sp_PersonNotesGet(#PersonID) which returns a set of related notes for this person.
So far so good, I have an object:
Dim myPerson As Person = db.PersonGet(pnID).Single
and I can access my fields: myPerson.Name, myPerson.Phone etc.
and I can also do a
Dim myNotes As Notes = db.PersonNotesGet(pnID)
to get a set of notes and I can iterate thru this list like:
For Each N As Note In myNotes
( do something)
Next
This all works fine...BUT....What I would prefer is that if I call:
myPerson = db.PersonGet(pnID)
that I also end up with a myPerson.Notes collection that I can iterate thru.
For Each N As Note In myPerson.Notes
( do something)
Next
Basically, Linq2SQl would need to call 2 stored procedures each time a Person record is created...
Is this doable "out of the box", or is this something I need to code around for myself?
This is normally what we would call child collections and they can be eager loaded or lazy loaded. Read these:
http://davidhayden.com/blog/dave/archive/2009/01/08/QuickExamplesLINQToSQLPerformanceTuningPerformanceProfilingORMapper.aspx
http://www.thinqlinq.com/default/Fetching-child-records-using-Stored-Procedures-with-LINQ-to-SQL.aspx
It uses partial classes. You can add your own "Notes" property to your Person class and initialize it in it's GETter function. This would be better than populating the notes every time you load a person record.
I believe that you can do this more or less out of the box, although I haven't tried it -- I don't use stored procedures with LINQ. What you would need to do is change the Insert/Delete/Update methods from using the runtime to use your stored procedures. Then you'd create an Association between your two entity tables which would create an EntitySet of Notes on the Person class and a EntityRef of Person on the Notes class. You can set this up to load automatically or using lazy loading.
The only tricky bit, as far as I can see, is the change from using the runtime generated methods to using your stored procedures. I believe that you have to add them into the data context as methods (by dropping it on your table from the server explorer in the designer) before it is available to use instead.