my database like;
I want, when announcement0 field is deleted, announcement1 field name to change announcement0. Is there a way to do this ?
There is no way to rename fields in Firestore, let alone to have that happen automatically.
It sounds like you have multiple announcements in your document however. In that case, you could consider storing all announcements in a single array field announcements. In an array field, when you remove the first item (at index 0) all other items after that shift down in the array to take its place, which seems to be precisely what you want.
You cannot rename fields in a document. You'll have to delete and recreate it.
Now I'm assuming the number just defines the order of document. If that's the case you can use this workaround, instead of looking for 'announcement0' on client side, you can just store a number field in the document such as 0 in announcement0 and so on. So to get announcement1 when announcement0 is deleted you can uses this query:
const firstAnnouncement = await dbRef.orderBy('number').limit(1).get()
This will get the announcement with least number (highest rank). You can change the limit as per your needs.
But if renaming fields is needed then you'll have to delete and recreate all trailing announcements.
Related
I'm developing an app which needs to record a list of a users recent video uploads. Importantly it needs to only remember the last two videos associated with the user so I'm trying to find a way to just keep the last two records in a database.
What I've got so far is the below, which creates a new record correctly, however I then want to delete all records that are older than the previous 2, so I've got the below.
The problem is that this seems to delete ALL records even though, by my understanding, the skip should miss out the two most recent records,
private function saveVideoToUserProfile($userId, $thumb ...)
{
RecentVideos::create([
'user_id'=>$userId,
'thumbnail'=>$thumb,
...
]);
RecentVideos::select('id')->where('user_id', $userId)->orderBy('created_at')->skip(2)->delete();
}
Can anyone see what I'm doing wrong?
Limit and offset do not work with delete, so you can do something like this:
$ids = RecentVideos::select('id')->where('user_id', $userId)->orderByDesc('created_at')->skip(2)->take(10000)->pluck('id');
RecentVideos::whereIn('id', $ids)->delete();
First off, skip() does not skip the x number of recent records, but rather the x number of records from the beginning of the result set. So in order to get your desired result, you need to sort the data in the correct order. orderBy() defaults to ordering ascending, but it accepts a second direction argument. Try orderBy('created_at', 'DESC'). (See the docs on orderBy().)
This is how I would recommend writing the query.
RecentVideos::where('user_id', $userId)->orderBy('created_at', 'DESC')->skip(2)->delete();
The following document records a conversation between Milhouse and Bart. I would like to insert a new message with the right num (the next in the example would be 3) in a unique operation. Is that possible ?
{ user_a:"Bart",
user_b:"Milhouse",
conversation:{
last_msg:2,
messages:[
{ from:"Bart",
msg:"Hello"
num:1
},
{ from:"Milhouse",
msg:"Wanna go out ?"
num:2
}
]
}
}
In MongoDB, arrays keep their order, so by adding a num attribute, you're only creating more data for something that you could accomplish without the additional field. Just use the position in the array to accomplish the same thing. Grabbing the X message in an array will provide faster searches than searching for { num: X }.
To keep the order, I don't think there's an easy way to add the num category besides does a find() on conversation.last_msg before you insert the new subdocument and increment last_msg.
Depending on what you need to keep the ordering for, you might consider including a time stamp in your subdocument, which is commonly kept in conversation records anyway and may provide other useful information.
Also, I haven't used it, but there's a Mongoose plugin that may or may not be able to do what you want: https://npmjs.org/package/mongoose-auto-increment
You can't create an auto increment field but you can use functions to generate and administrate sequence :
http://docs.mongodb.org/manual/tutorial/create-an-auto-incrementing-field/
I would recommend using a timestamp rather than a numerical value. By using a timestamp, you can keep the ordering of the subdocument and make other use of it.
I have a listing site having a model with many properties on which I would like to use filters. I would like to use memcache and cursor for querying, e.g:
results=Model.all().filter("x =", a).filter("y =",b).with_cursor(cursor).fetch(20).
How should I handle cursor and pagination, when the user change the filter criteria, e.g.
from `x=a to x=c`?
Should I store cursor having key = query string? But then the query string changes with page numbers :( . I guess i will need to parse query string, remove page numbers and use that as a key for cursor. Is that how I should do it?
You can make an "hash" of your current filter, and pass it to view. That can be stores there as an hidden field, like <input type="hidden" name="prev_query" value="{{query_hash}}"/>
On second request you'll check that current filter's hash equals to passed as parameter.
'Hash' maybe md5 of your filter params, or just join concatenation of them.
Think of a cursor like a bookmark, holding place in a result set. Cursors are specific to the query they are for. You can't use the same cursor for two different queries - that would be akin to expecting a bookmark from one book to show you how far through you are in another.
If you want to store cursors elsewhere, you'll need to key them by the filter criteria so you can look up the appropriate one. Memcache is a poor choice, though, as elements may be evicted at any time. Why not just make the cursor part of the 'next page' URL?
I want to retrieve the last row from the data store so how can i do that??
I know the long method i.e
for(Table_name e: resultset)
{
cnt++;
}
results.get(cnt).getvalue();
I have String with (number) as primary key.can i use it to get descending order???
Is there any method through which i can get the last row???
You should probably sort in the opposite order (if possible for your query, the data store has some restrictions here) and get the first result of that.
Also, if you store numbers in String fields the order may not be what you want it to be (you might need padding here).
I am trying a case where we changed a field name in our entity. we have something like this for example
class Person {
String name; //The original declaration was "String fullName"
}
According to objectify you have to use annonation #AutoLoad(""). This is ok and it works as Google Datastore doesn't delete the data Actually but it makes a new field so this annotation is like a mapping between the old and the new field. No problem when you are reading the whole table.
The problem arises when you apply a filter on your query (Suppose you made 5 objects with old name and 5 with new name). The result of your query depends on whether you used the old variable name or the new one (returns back only 5 but never the 10). It won't fetch both of them and map them. Any Suggestions for this problem? I hope i explained it in a clear way.
Thanks in advance
The simplest straight forward solution. fetch all data with the annonation "AutoLoad()". Then store them again. In this way they will be saved as the new field. The old one doesn't exist anymore or at least it doesn't contain any data anymore. It is like migrating the data from the old name to the new name. Anyone has better suggestions ?
If you've changed the name of your field, you need to load and re-put all your data (using the mapreduce API would be one option here). There's no magic way around this - the data you've stored exists with two different names on disk.
You can use #OldName
http://www.mail-archive.com/google-appengine-java#googlegroups.com/msg05586.html