So realm now has maps, called RealmDictionary. Great. But can I sort RealmResults by a value that is inside an entry with a specific key?
Related
I have a use case where DynamoDB is running in production and I need to add a new column IDUpdatedAt which will also be serving as a sort key for one of the GSIs.
I tried a thing in test where my application adds the new rows with IDUpdatedAt, it's working fine but what about the existing rows? How to add the values for those?
Also the new rows will not be added without IDUpdatedAt, but how will the search be impacted for older rows?
PS: IDUpdatedAt is being used as a filter in the application, i.e., user can search for specific ID and can get results sorted by date. That's why IDUpdatedAt is also a part of GSI (sort key).
Please help.
You've got the right idea by adding the field to new items. After all, DynamoDB does not enforce a particular schema outside of the primary key.
This also happens to be a very useful feature, especially when defining a GSI on that attribute; if the atttibute exists on the item, it ends up in the index! For example, imagine modeling an email inbox in DDB where each item represents an email. You could include an attribute 'is_read' and define a GSI using that atttibute.
If the 'is_read' attribute exists on the item, it's in the index. Otherwise, it's not. A cool way to use GSIs to implement filtering.
Pretty neat stuff!
However, there is no way to retroactively update all items with a new attribute other than manually updating each item (or in batches). The equivalent in SQL databases is defining a new column. Unfortunately, an analogous operation in DDB does not exist.
I have a button which runs the following code when it's clicked:
let dataReference = await db.collection("dog").doc("1").get()
let HashMap = dataReference.data().Annotations
console.log(HashMap)
My firestore database looks like this:
Whenever this function is run, it returns the proper dictionary, however, the ordering of the keys seems to change randomly. Here's a screenshot of my console logs when I pressed the button a bunch of times:
Why does the ordering of the key-value pairs change and is there a way to fix it?
The Firestore SDK does not guarantee an order of iteration of document fields. What you see in the console is always lexically sorted by the code of the console itself. If you require a stable ordering, you should sort them yourself before iteration.
One workaround you can do is to have the order of the keys you want in an array. Because arrays are ordered it will maintain the order you desired. Then you take that key and use it in the dictionary. While the dictionary is out of order, you will be accessing each value in order by key.
I'm quite new to DynamoDB, but have some experience in Cassandra. I'm trying to adapt a pattern I followed in Cassandra, where each column represented a timestamped event, and wondering if it will carry over gracefully into DynamoDB or if I need to change my approach.
My goal is to query a set of documents within a date range by using the milliseconds-since-epoch timestamp as an Attribute name. I'm successfully storing the following as each report is generated with each new report being added under its own column:
{ PartitionKey:customerId,
SortKey:reportName_yyyymm,
'#millis_1#':{'report':doc_1},
'#millis_2#':{'report':doc_2},
. . .
'#millis_n#':{'report':doc_n}
}
My question is, given a millisecond-based date range, and the accompanying Partition and Sort keys, is it possible to query the set of Attributes that fall within that range or must I retrieve all columns for the matching keys and filter them at the client?
Welcome to the most powerful NoSQL database ;)
To kick off with the positive news, there is no way to query out specific attributes. You can project certain attributes in a query. But you would have to write your own logic to determine which attributes or columns should be included in the projected query. To get close to your solution you could use a map attribute inside an item with the milliseconds as a key. But there is another thing you have to be aware of when starting on this path.
There is a maximum total item size of 400KB for each item in DynamoDB, including key and attribute names.(Limits in DynamoDB Items) This means you can only store so many attributes in an item. This is especially true if you intend to put the actual report inside of the attribute. Which I would advise against, also because you will be burning up read capacity units every time you get one attribute out of the whole item. You would be better of putting this data in a separate table with the keys in the map. But truthfully in DynamoDB I would split this whole thing up, just add the milliseconds to the sort key and make every document its own item. That way you can directly query to these items and you can use the "between" where clause to select specific date-time ranges. Please let me you meant something else.
I have a root in firebase like in the I am trying to enable user to delete an item on list. But user can give up his decision. When user give up this decision, I want to insert the deleted item again in the database. But, I want to insert with old firebase generated key, because I am using firebase push keys. Is that a bad practice. How firebase generate these keys? Does it checks every key on db and generate a new one? Is that any possibility, that key marked as removed and generated later for another item? Sorry for the language. It has been hard to express.
EDITED: I want to use the old key because, I am getting the data with orderByKey. I dont want to lose order.
How firebase generate these keys? Does it checks every key on db and generate a new one?
Whenever you use push on a Database Reference, a new data node is generated with a unique key that includes the server timestamp. These keys look like -KiGh_31GA20KabpZBfa.
Because of the timestamp, you can be sure that the given key will be unique, without having to check the other keys inside your database.
Is that any possibility, that key marked as removed and generated later for another item?
No, it is not possible that two keys will collide, regardless of wether one has been removed or not.
But, I want to insert with old firebase generated key, because I am using firebase push keys. Is that a bad practice
Unfortunately, you can't generate the same key twice by just using push. So, it is not possible to delete a node with a given key and then use push to insert it again at the same path with the same key, because push would generate a different and unique key.
Instead of this, if ordering by key is that important to you, and there's a possibility that a deleted node can be reinserted then I would recommend you to do one of the following :-
Either save the key on the client side when it's deleted from the database, and use it when you need to reinsert.
Or , maybe, have a "deleted-keys" path in your database and save the deleted keys there. Of course, with this approach, you'd need to store additional information to identify the data that the key corresponds to.
It all really depends on your use case.
Calling push() will generate a key for you.
If instead you use child(), you can determine they key/path yourself.
ref.child("yourvalue").setValue("setting custom key when pushing new data to firebase database");
https://firebase.googleblog.com/2015/02/the-2120-ways-to-ensure-unique_68.html
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.