I've read multiple questions in Stack Overflow and the documentation but I couldn't find multiple validations that I can imagine that exist.
Per example, it is possible to check if request.resource.data.description is string but can is it possible to do the same when it comes to number, float, timestamp or even array/list? I couldn't even find the string one in the documentation so I can imagine it is missing more than just that one.
You might want to watch my video on data types in Firebase security rules. In it, I list all the different data types that you can check:
value is bool
value is int
value is float
value is number
value is string
value is list
value is map
value is timestamp
value is duration
value is path
value is latlng
Related
I have a DDB table, 4 attributes, key (PK - a string), date (sort/range key), status, frequency.
I have multiple clients that will write to this table based on the 'key' and date value
I want to increment frequency every time a client makes a write.
Can I just use DynamoDBVersionAttribute on an int field and use this as a proxy for frequency?
I understand this is not meant for this use case, but I want to avoid having to first read and then write the item. Any thoughts?
Since you're already doing an update expression, just add an ADD action to increment the frequency by 1. The ADD action doesn't need to know the original value to increment it.
See the example from the docs here:
https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/Expressions.UpdateExpressions.html#Expressions.UpdateExpressions.SET.IncrementAndDecrement
Recently Freebase added flags for values: Has No Values and Has Unknown Values. If I query a property's value it will give me null if it is empty or has one of these flags. How do I determine if the value really is empty or if it's flagged?
A good place to find out about something that you see on freebase.com is the source code that drives the site. In particular, for this case:
https://code.google.com/p/freebase-site/source/browse/trunk/www/lib/propbox/js/propbox-edit.js#640
The properties that you need to query are:
/freebase/valuenotation/has_no_value
/freebase/valuenotation/has_value
Here's an example:
https://www.freebase.com/m/05r3dj?links&lang=en&filter=%2Ffreebase%2Fvaluenotation%2Fhas_value
I have a JDOQL/DataNucleus storage layer which stores values that can have multiple primitive types in a varchar field. Some of them are numeric, and I need to compare (</>/...) them with numeric constants. How does one achieve that? I was trying to use e.g. (java.lang.)Long.parse on the field or value (e.g. java.lang.Long.parseLong(field) > java.lang.Long.parseLong(string_param)), supplying a parameter of type long against string field, etc. but it doesn't work. In fact, I very rarely get any errors, for various combinations it would return all values or no values for no easily discernible reasons.
Is there documentation for this?
Clarification: the field is of string type (actually a string collection from which I do a get). For some subset of values they may store ints, e.g. "3" string, and I need to do e.g. value >= 2 filters.
I tried using casts, but not much, they do produce errors, let me investigate some more
JDO has a well documented set of methods that are valid for use with JDOQL, upon which DataNucleus JDO adds some additional ones and allows users to add on support for others as per
http://www.datanucleus.org/products/accessplatform_3_3/jdo/jdoql.html#methods
then you also can use JDOQL casts (on the same page as that link).
I have a lot of objects with unique IDs. Every object can have several labels associated to it, like this:
123: ['a', 'hello']
456: ['dsajdaskldjs']
789: (no labels associated yet)
I'm not planning to store all objects in DynamoDB, only these sets of labels. So it would make sense to add labels like that:
find a record with (id = needed_id)
if there is one, and it has a set named label_set, add a label to this set
if there is no record with such id, or the existing record doesn't have an attribute named label_set, create a record and an attribute, and initialize the attribute with a set consisting of the label
if I used sets of numbers, I could use just ADD operation of UPDATE command. This command does exactly what I described. However, this does not work with sets of strings:
If no item matches the specified primary key:
ADD— Creates an item with supplied primary key and number (or set of numbers) for the attribute value. Not valid for a string type.
so I have to use a PUT operation with Expected set to {"label_set":{"Exists":false}}, followed (in case it fails) by an ADD operation. These are two operations, and it kinda sucks (since you pay per operation, the costs of this will be 2 times more than they could be).
This limitations seems really weird to me. Why are something what works with numbers sets would not work with string sets? Maybe I'm doing something wrong.
Using many records like (123, 'a'), (123, 'hello') instead of one record per object with a set is not a solutions: I want to get all the values from the set at once, without any scans.
I use string sets from the Java SDK the way you describe all the time and it works for me. Perhaps it has changed? I basically follow the pattern in this doc:
http://docs.amazonwebservices.com/amazondynamodb/latest/developerguide/API_UpdateItem.html
ADD— Only use the add action for numbers or if the target attribute is
a set (including string sets). ADD does not work if the target
attribute is a single string value or a scalar binary value. The
specified value is added to a numeric value (incrementing or
decrementing the existing numeric value) or added as an additional
value in a string set. If a set of values is specified, the values are
added to the existing set. For example if the original set is [1,2]
and supplied value is [3], then after the add operation the set is
[1,2,3], not [4,5]. 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.
If you use ADD for an attribute that does not exist, the attribute and
its values are added to the item.
When your set is empty, it means the attribute isn't present. You can still ADD to it. In fact, a pattern that I've found useful is to simply ADD without even checking for the item. If it doesn't exist, it will create a new item using the specified key and create the attribute set with the value(s) I am adding. If the item exists but the attribute doesn't, it creates the attribute set and adds the value(s). If they both exist, it just adds the value(s).
The only piece that caught me up at first was that the value I had to add was a SS (String set) even if it was only one string value. From DynamoDB's perspective, you are always merging sets, even if the existing set is an empty set (missing) or the new set only contains one value.
IMO, from the way you've described your intent, you would be better off not specifying an existing condition at all. You are having to do two steps because you are enforcing two different situations but you are trying to perform the same action in both. So might as well just blindly add the label and let DynamoDB handle the rest.
Maybe you could: (pseudo code)
try:
add_with_update_item(hash_key=42, "label")
except:
element = new Element(hash_key=42, labels=["label"])
element.save()
With this graceful recovery approach, you need 1 call in the general case, 2 otherwise.
You are unable to use sets to do what you want because Dynamo Db doesn't support empty sets. I would suggest just using a string with a custom schema and building the set from that yourself.
To avoid two operations, you can add a "ConditionExpression" to your item.
For example, add this field/value to your item:
"ConditionExpression": "attribute_not_exists(RecordID) and attribute_not_exists(label_set)"
Source documentation.
Edit: I found a really good guide about how to use the conditional statements
Hello friends i am to add optional parameter to pass these in a method the parameter may vary every time.
And i need to pass these parameter to a method, so my question is how should i add these optional parameter to a collection and what kind of collection object should i use and how should i use that.
i elaborate here, i am having following fields.
1)course field(a drop down list) and on selected index change of course a check box list of corresponding branches are visible now i can select branches of choice by putting check mark on check boxes.(both course and branch are compulsory fields)
2)pass_out year which is a compulsory field.(a text field)
3)education gap which is optional so the text box may be empty as well and may having a digit as well.
4)required first year percent, second year, third year, final year, current degree, high school, senior secondary, gradation, post graduation, diploma percent, birth date which are all optional so these fields may also be empty or having a digit in the text box.
5) current backlog(a drop down list)having choices all, yes , no.
6)number of ATKT text box which may also be empty or can have a digit.
So i want to store all these variables value to some collection of object which will passed to a different method by passing all these variable from here,
But i am unable to figure out how should i store these optional variable to some object and how should i pass them to other method where all variable values will be taken out and an appropriate query will be written to interact with the data base to get the data table.
please elucidate me on this please. I am really not getting a feasible solution, i have thought of few options which i can let you know if you demand but those all seems to be tedious and difficult and redundant so i see help from you.
Don't bother with optional parameters or some collection of parameters, just decide how you will represent each value best, including how to represent empty values.
For a string value you can just use a null reference or an empty string. For a numeric value you can use a nullable int (int?). For a list of checkboxes where none is selected you can send an empty list.
When figuring out how to use this in a query to the database, it's easier if you have the values as parameters that always exist even if the values represent an empty value, rather than to have to parse out parameters from a collection where a parameter might be missing. Also, as you have different data types it's better if you can use an appropriate data type for each parameter, instead of casting everything into the same mold just to fit in a collection.