I've read some sample code (especially from the Couchbase Model Views demo project link) and realized the map() function is so strange.
function(doc, meta) {
if (doc.type == "beer" && doc.name){
emit(doc.name, null);
}
}
Why the emit function's value is null, but the result from GetView("beers", "beer") is getting the value perfectly???
Please help me out !
In couchbase usally view's result set is built in background. If you have for example 1 million documents each 4Kb size without any views it take ~4Gb on disk. When you create a view with map function like
function(doc, meta) {
emit(doc.name, doc);
}
As a result it take additional 4Gb on disk for view results because view results are stored separately. And in most cases (if you query view with param Stale=Ok) couchbase returns result from that "precompiled" set of records, couchbase doesn't scan all docs on each query.
So emmiting null in map functions is used to prevent disk space usage and it also increase speed of indexing process.
Now the second question about couchbase magic when "result from GetView("beers", "beer") is getting the value perfectly". Couchbase get(key) and getMulti(keys) operations are very fast. So when you query view that emits null it returns not only nulls, also it returns document ids. Then you can manually use getMulti for that array of document ids to get doc's value or in some SDKs there is query param called IncludeDocs that will do the same automatically.
Related
I have a code in to get all data in one collection in cosmo db, if the collection is empty then start to inserting.
The first time the code is run collection is empty however, SetIterator.HasMoreResults return True, even though the collection is empty.
Then there will be error raising Microsoft.Azure.Cosmos.CosmosException : Response status code does not indicate success: NotFound, and I have checked with a collection that is not empty the code run fine.
I can use try catch to handle it, but it does not seems to be a nice solution, Does anyone know how to check if the collection is empty?
var itemList = new List<T>();
using (FeedIterator<T> setIterator = _container.GetItemLinqQueryable<T>()
.ToFeedIterator())
{
while (setIterator.HasMoreResults)
{
foreach (var item in await setIterator.ReadNextAsync())
{
itemList.Add(item);
}
}
}
return itemList;
You can use indexing to get empty collection in cosmos db. To do this you can use unique indexes. Unique indexes are created to only those which doesn’t contain any documents.
Important
Unique indexes can be created only when the collection is empty
(contains no documents).
To create unique indexes on collections you can go through unique indexes
To optimize usage, I have a Firestore collection with only one document, consisting in a single field, which is an array of strings.
This is what the data looks like in the collection. Just one document with one field, which is an array:
On the client side, the app is simply retrieving the entire status document, picking one at random, and then sending the entire array back minus the one it picked
var all = await metaRef.doc("status").get();
List tokens=all['all'];
var r=new Random();
int numar=r.nextInt(tokens.length);
var ales=tokens[numar];
tokens.removeAt(numar);
metaRef.doc("status").set({"all":tokens});
Then it tries to do some stuff with the string, which may fail or succeed. If it succeeds, then no more writing to the database, but if it fails it fetches that array again, adds the string back and pushes it:
var all = await metaRef.doc("status").get();
List tokens=all['all'];
List<String> toate=(tokens.map((element) => element as String).toList());
toate.add(ales.toString());
metaRef.doc("status").set({"all":toate});
You can use the methods associated with the Set object.
Here is an example to check that only 1 item was removed:
allow update: if checkremoveonlyoneitem()
function checkremoveonlyoneitem() {
let set = resource.data.array.toSet();
let setafter = request.resource.data.array.toSet();
return set.size() == setafter.size() + 1
&& set.intersection(setafter).size() == 1;
}
Then you can check that only one item was added. And you should also add additional checks in case the array does not exist on your doc.
If you are not sure about how the app performs the task i.e., successfully or not, then I guess it is nice idea to implement this logic in the client code. You can just make a simple conditional block which deletes the field from the document if the operation succeeds, either due to offline condition or any other issue. You can find the following sample from the following document regarding how to do it. Like this, with just one write you can delete the field which the user picks without updating the whole document.
city_ref = db.collection(u'cities').document(u'BJ')
city_ref.update({
u'capital': firestore.DELETE_FIELD
})snippets.py
I am fairly new in this realm and any help is appreciated
I have a table in Dynamodb database named Tenant as below:
"TenantId" is the hash primary key and I have no other keys. And I have a field named "IsDeleted" which is boolean
Table Structure
I am trying to run a query to get the record with specified "TenantId" while it is not deleted ("IsDeleted == 0")
I can get a correct result by running the following code: (returns 0 item)
var filter = new QueryFilter("TenantId", QueryOperator.Equal, "2235ed82-41ec-42b2-bd1c-d94fba2cf9cc");
filter.AddCondition("IsDeleted", QueryOperator.Equal, 0);
var dbTenant = await
_genericRepository.FromQueryAsync(new QueryOperationConfig
{
Filter = filter
}).GetRemainingAsync();
But no luck when I try to get it with following code snippet (It returns the item which is also deleted) (returns 1 item)
var queryFilter = new List<ScanCondition>();
var scanCondition = new ScanCondition("IsDeleted", ScanOperator.Equal, new object[]{0});
queryFilter.Add(scanCondition);
var dbTenant2 = await
_genericRepository.LoadAsync("2235ed82-41ec-42b2-bd1c-d94fba2cf9cc", new DynamoDBOperationConfig
{
QueryFilter = queryFilter,
ConditionalOperator = ConditionalOperatorValues.And
});
Any Idea why ScanCondition has no effect?
Later I also tried this: (throw exception)
var dbTenant2 = await
_genericRepository.QueryAsync("2235ed82-41ec-42b2-bd1c-d94fba2cf9cc", new DynamoDBOperationConfig()
{
QueryFilter = new List<ScanCondition>()
{
new ScanCondition("IsDeleted", ScanOperator.Equal, 0)
}
}).GetRemainingAsync();
It throws with: "Message": "Must have one range key or a GSI index defined for the table Tenants"
Why does it complain about Range key or Index? I'm calling
public AsyncSearch<T> QueryAsync<T>(object hashKeyValue, DynamoDBOperationConfig operationConfig = null);
You simply cant query a table only giving a single primary key (only hash key). Because there is one and only one item for that primary key. The result of the Query would be that still that single item, which is actually Load operation not Query. You can only query if you have composite primary key in this case (Hash (TenantID) and Range Key) or GSI (which doesn't impose key uniqueness therefore accepts duplicate keys on index).
The second code attempts to filter the Load. DynamoDBOperationConfig's QueryFilter has a description ...
// Summary:
// Query filter for the Query operation operation. Evaluates the query results and
// returns only the matching values. If you specify more than one condition, then
// by default all of the conditions must evaluate to true. To match only some conditions,
// set ConditionalOperator to Or. Note: Conditions must be against non-key properties.
So works only with Query operations
Edit: So after reading your comments on this...
I dont think there conditional expressions are for read operations. AWS documents indicates they are for put or update operations. However, not being entirely sure on this since I never needed to do a conditional Load. There is no such thing like CheckIfExists functionality as well in general. You have to read the item and see if it exists. Conditional load will still consume read throughput so your only advantage would be only NOT retrieving it in other words saving the bandwith (which is very negligible for single item).
My suggestion is read it and filter it in your application layer. Dont query for it. However what you can also do is if you very need it you can use TenantId as hashkey and isDeleted for range key. If you do so, you always have to query when you wanna get a tenant. With the query you can set rangeKey(isDeleted) to 0 or 1. This isnt how I would do it. As I said, would just read it and filter it at my application.
Another suggestion thing could be setting a GSI on isDeleted field and writing null when it is 0. This way you can only see that attribute in your table when its only 1. GSI on such attribute is called sparse index. Later if you need to get all the tenants that are deleted (isDeleted=1) you can simply scan that entire index without conditions. When you are writing null when its 0 dynamoDB wont put it in the index at the first place.
I want select from Firestore collection just articles written NOT by me.
Is it really so hard?
Every article has field "owner_uid".
Thats it: I JUST want to write equivalent to "select * from articles where uid<>request.auth.uid"
TL;DR: solution found already: usages for languages/platforms: https://firebase.google.com/docs/firestore/query-data/queries#kotlin+ktx_5
EDIT Sep 18 2020
The Firebase release notes suggest there are now not-in and != queries. (Proper documentation is now available.)
not-in finds documents where a specified field’s value is not in a specified array.
!= finds documents where a specified field's value does not equal the specified value.
Neither query operator will match documents where the specified field is not present. Be sure the see the documentation for the syntax for your language.
ORIGINAL ANSWER
Firestore doesn't provide inequality checks. According to the documentation:
The where() method takes three parameters: a field to filter on, a comparison operation, and a value. The comparison can be <, <=, ==, >, or >=.
Inequality operations don't scale like other operations that use an index. Firestore indexes are good for range queries. With this type of index, for an inequality query, the backend would still have to scan every document in the collection in order to come up with results, and that's extremely bad for performance when the number of documents grows large.
If you need to filter your results to remove particular items, you can still do that locally.
You also have the option of using multiple queries to exclude a distinct value. Something like this, if you want everything except 12. Query for value < 12, then query for value > 12, then merge the results in the client.
For android it should be easy implement with Task Api.
Newbie example:
FirebaseFirestore db = FirebaseFirestore.getInstance();
Query lessQuery = db.collection("users").whereLessThan("uid", currentUid);
Query greaterQuery = db.collection("users").whereGreaterThan("uid", currentUid);
Task lessQuery Task = firstQuery.get();
Task greaterQuery = secondQuery.get();
Task combinedTask = Tasks.whenAllSuccess(lessQuery , greaterQuery)
.addOnSuccessListener(new OnSuccessListener<List<Object>>() {
#Override
public void onSuccess(List<Object> list) {
//This is the list of "users" collection without user with currentUid
}
});
Also, with this you can combine any set of queries.
For web there is rxfire
This is an example of how I solved the problem in JavaScript:
let articlesToDisplay = await db
.collection('articles')
.get()
.then((snapshot) => {
let notMyArticles = snapshot.docs.filter( (article) =>
article.data().owner_uid !== request.auth.uid
)
return notMyArticles
})
It fetches all documents and uses Array.prototype.filter() to filter out the ones you don't want. This can be run server-side or client-side.
Updating the answer of Darren G, which caused "TypeError: Converting circular structure to JSON". When we perform the filter operation, the whole firebase object was added back to the array instead of just the data. We can solve this by chaining the filter method with the map method.
let articles = []
let articlesRefs = await db.collection('articles').get();
articles = articlesRefs.docs
.filter((article) => article.data.uid !== request.auth.uid) //Get Filtered Docs
.map((article) => article.data()); //Process Docs to Data
return articles
FYI: This is an expensive operation because you will fetching all the articles from database and then filtering them locallly.
Track all user id in a single document (or two)
filter unwanted id out
Use "where in"
var mylistofidwherenotme = // code to fetch the single document where you tracked all user id, then filter yourself out
database.collection("articles").where("blogId", "in", mylistofidwherenotme)
let query = docRef.where('role','>',user_role).where('role','<',user_role).get()
This is not functioning as the "not equal" operation in firestore with string values
You can filter the array of objects within the javascript code.
var data=[Object,Object,Object] // this is your object array
var newArray = data.filter(function(el) {
return el.gender != 'Male';
});
I got the following issue. I am trying to use the With or WithMany instruction
to retrieve a linked list of roles of an business relation via an
outer join. The referential integrity is in place on the database but
the primary key on the roles table is a composite key. That's the
reason i use an OuterJoin clause because I get an exception
otherwise .
When the query gets executed the results are exactly as I expected and
nicely filled with data. Nevertheless there are certain cases where
there are not yet roles available in the database. So I would expect
that in those cases the returned SimpleList (Roles below) would be
null, cause there is not data available. Instead Simple.Data returns a
SimpleLIst and if i expand the Dynamic View in debug then it says
'Empty: No further information on this object could be discovered".
Even if i traverse further down and i retrieve the first object in the
SimpleList, it even returns a SimpleRecord with the same information
as above in debug. Only after I request a property of the SimpleRecord
I get some information that the record was empty because then it
returns null.
To come to the bottom line... is there anybody who can tell me how to
check if a SimpleList or SimpleRecord is empty or null without
traversing down the hierarchy?
I am using Simple.Data 0.16.1.0 (due to policy i can't use the
beta yet)
Thanks in advance for reading the whole story...
Below is the code sample:
dynamic businessRelationRoles;
var query = db.Zakenrelaties.As("BusinessRelations")
.All()
.OuterJoin(db.Zakenrelaties_Rollen.As("Roles"), out businessRelationRoles)
.On(zr_ID: db.Zakenrelaties.zr_ID)
.With(businessRelationRoles);
foreach (var relation in query)
{
//Get the SimpleList as IEnumerable
IEnumerable<dynamic> roles = relation.Roles;
//Get the first available SimpleRecord
var role = roles.First();
//Check if any record was returned..This passes always?? Even if the SimpleList was empty
if (role != null)
{
//Get the id of the role. returns null if SimpleRecord was empty
var roleId = role.zrro_id;
}
}
Is there anybody who can help me out?
Belatedly, and for information purposes only, this was a bug and got fixed in the 0.17 (aka 1.0-RC0) release.