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.
Related
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.
Please I want to give custom IDs to my database objects in Firebase but I don't now how to do it. Firebase creates default IDs for database objects which I don't want. I want to be able to assign my own IDs to objects or the child nodes of in the database for unique identification.
Most likely you're adding the items to the database with something like:
ref.push().set("my value");
This generates a new unique key under ref and sets your value on it.
If you want to use you own key/name for the child location, add the item with:
ref.child("my key").set("my value");
You cannot customize ID of firebase object, but you can create another field with ID role.
ref.child("my_id").set("customize_id");
after that, using "Filter by key" to get exactly your object you want.
In our case: We need to have a user_id type Int and auto-increase, so we can't use default _id of firebase object, we create user_id ourself to solve this problem.
I'm working on a website that mostly displays items created by registered users. So I'd say 95% of API calls are to read a single item and 5% are to store a single item. System is designed with AWS API Gateway that calls AWS Lambda function which manipulates data in DynamoDB.
My next step is to implement voting system (upvote/downvote) with basic fetaures:
Each registered user can vote only once per item, and later is only allowed to change that vote.
number of votes needs to be displayed to all users next to every item.
items have only single-item views, and are (almost) never displayed in a list view.
only list view I need is "top 100 items by votes" but it is ok to calculate this once per day and serve cached version
My goal is to design a database/lambda to minimize costs of AWS. It's easy to make the logic work but I'm not sure if my solution is the optimal one:
My items table currently has hashkey slug and sortkey version
I created items-votes table with hashkey slug and sortkey user and also voted field (containing -1 or 1)
I added field votes to items table
API call to upvote/downvote inserts to item-votes table but before checks constraints that user has not already voted that way. Then in second query updates items table with updated votes count. (so 1 API call and 2 db queries)
old API call to show an item stays the same but grabs new votes count too (1 API call and 1 db query)
I was wondering if this can be done even better with avoiding new items-votes table and storing user votes inside items table? It looks like it is possible to save one query that way, and half the lambda execution time but I'm worried it might make that table too big/complex. Each user field is a 10 chars user ID so if item gets thousands of votes I'm not sure how Lambda/DynamoDB will behave compared to original solution.
I don't expect thousands of votes any time soon, but it is not impossible to happen to a few items and I'd like to avoid situation where I need to migrate to different solution in the near future.
I would suggest to have a SET DynamoDB (i.e. SS) attribute to maintain the list of users who voted against the item. Something like below:-
upvotes : ['user1', 'user2']
downvotes : ['user1', 'user2']
When you update the votes using UpdateExpression, you can use ADD operator which adds users to SET only if it doesn't exists.
ADD - Adds the specified value to the item, if the attribute does not
already exist. If the attribute does exist, then the behavior of ADD
depends on the data type of the attribute:
If the existing data type is a set and if Value is also a set, then
Value is added to the existing set. For example, if the attribute
value is the set [1,2], and the ADD action specified [3], then the
final attribute value is [1,2,3]. An error occurs if an ADD action is
specified for a set attribute and the attribute type specified does
not match the existing set type. Both sets must have the same
primitive data type. For example, if the existing data type is a set
of strings, the Value must also be a set of strings.
This way you don't need to check whether the user already upvote or downvote for the item or not.
Only thing you may need to ensure is that the same user shouldn't be present on upvote and downvote set. Probably, you can use REMOVE or ConditionExpression to achieve this.
I'm using ElasticaBundle and ElasticSearch with Symfony2 in a system I've written.
A 'person' can have many 'positions' in their work history. The positions are sorted by date desc, and in order to find someone's current position with PHP I retrieve and read the first object in the array.
I am struggling to search only the current or first position using ElasticSearch. I have set the mappings upas nested, and I am able to perform a Nested Query returning a 'person' who has a 'position' that matches all my criteria. What I can't do is work out how to only search for the criteria in the 1st listed 'position'. Does anyone have any ideas to set me off on the right path?
The only options I can think of at the moment are:
maintain an order value in each object so I can pick out the 1st,
or create another field in the entity that only has a relationship, with the 1st position
I've read on the ElasticSearch documentation that this kind of key isn't supported in ElasticSearch, but now I can't find the page in question. Sorry.
In the end I got round by the problem by setting an identifier on the current position, i.e. giving it a identifier of 0 with all other positions numbers in order.
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.