I've seen a lot of examples of using UpdateExpression to update attributes using the updateItem method. However, I still don't understand how to update multiple attributes in DynamoDB at the same time dynamically.
I am trying to update AND rename multiple attributes in the same updateItem call.
I understand that this requires a REMOVE of the old name and a SET of the new name. I have these names in hashedId's of objects, but won't have them until runtime. So my question is how do I use UpdateExpression with variables and not a hard-coded String?
All the examples I have seen use hard-coded UpdateExpressions.
can't update item in DynamoDB
Dynamo DB : UpdateItemSpec : Multiple Update Expression - Not Working
DynamoDB update Item multi action
How to rename DynamoDB column/key
I am working in Java.
It seems very odd to me that I haven't been able to find an example of this... which leads me to believe I am doing something wrong.
Thanks for the help!
You have to build the update expression string dynamically based on the attribute names and values that you receive at runtime. I do exactly this. I'm not working in Java, but here is some pseudo code (with a Ruby bias) example for you that dynamically builds the update expression string, the expression attribute names hash, and the expression attribute values hash. You can then plug in these 3 things into the update_item method:
update_exp_set = [] //array of remove expression snippets
update_exp_remove = [] //array of update expression snippets
exp_attribute_names = {} //hash of attribute names
exp_attribute_values = {} //hash of attribute values
// Iterate through all your fields and add things as needed to your arrays and hashes.
// Two examples are below for a field with the name <fieldName> and value <fieldValue>.
// You'll need to convert this to Java and update it to make sense with the AWS Java SDK.
// For a given field that needs to be updated:
update_exp_set << "#<fieldName> = :fieldValue" //add to array of set expression snippets
exp_attribute_names["#<fieldName>"] = "<fieldName>" //add to hash of attribute names
exp_attribute_values[":<fieldValue>"] = "<fieldValue>" //add to hash of attribute values
// For a given field that needs to be removed:
update_exp_remove << "#<fieldName>"
exp_attribute_names["#<fieldName>"] = "<fieldName>" //add to hash of attribute names
// Use your snippets to create your full update expression:
update_exp_set_clause = ""
update_exp_remove_clause = ""
if update_exp_set.length != 0 //check if you have something to set
update_exp_set_clause = "SET " + update_exp_set.join(',')
end
if update_exp_remove.length != 0 //check if you have something to remove
update_exp_remove_clause = "REMOVE" + update_exp_remove.join(',')
end
final_update_exp = update_exp_set_clause + " " + update_exp_remove_clause
Does this help?
Related
I am using Dynamodb Item - getItem API to get records from DynamoDB table. But it returns Item object and I want to retrieve specific attribute value from the Item object. How can we do it in Java? I couldn't find references.
Table table = dynamoDB.getTable(tableName);
Item item = table.getItem(hashKeyFieldName, hashKeyFieldValue);
The item contains the following fields:
HashKey, TimeStamp, NumRetries
I want to get the specific NumRetries value from item above. Is it something that is possible? something like int numRetries = item.get("NumRetries");?
You should be able to do that with a Projection Expression:
GetItemSpec spec = new GetItemSpec().withPrimaryKey("primaryKey", primaryKey)
.withProjectionExpression("HashKey, TimeStamp, NumRetries");
Item outcome = table.getItem(spec);
A names map may be necessary.
You can use Projection Expressions to get certain attributes from an item but do keep in mind that using projection expressions does not reduce the usage and cost of RCUs that are used in retrieving the object.
Code example,
GetItemSpec spec = new GetItemSpec()
.withPrimaryKey("YourPrimaryKey", value)
.withProjectionExpression("NumRetries");
Item item = table.getItem(spec);
System.out.println(item.toJSONPretty());
More code examples can be found here.
I have a use case as below.
List<Document> from mongo
For each Document
Get a field
Add to a set
Alter the field
Check altered field against two other sets
If not present convert to a pojo
add pojo to a list
I have something as below.
Optional.ofNullable(documentList).orElseGet(Collections::emptyList).forEach(doc -> {
// I am doing all the operations in java7 way
})
I thought something similar as below.
Iterate document list - add field to the set
Iterate the set from step1 - check field not contains on two other collections - then form pojo from original document - add pojo to list
But here I need to iterate twice. Moreover I need to maintain my original doc also. If my list is huge, iteration time is added unnecessarily.
How can I do this in standard way?
documentList.forEach(doc -> {
String deviceId = doc.getString("deviceId");
deviceSet.add(deviceId);
String alteredId = "IMEI" + deviceId;
if(!clearedDevices.contains(alteredId) && !superSetDevices.contains(deviceId)) {
//Pojo from Document
//add pojo to a list
}
})
Well it depends on the type of the concerned field. If it is a mutable object, then Java 8 stream is perfect. Something like:
documentList.stream()
.map(d->d.getString("deviceId") // stream of Stringified field
.peek(deviceSet::add) // add field to set
.map("IMEI"::concat) // alter the field
.filter(mf->!clearedDevices.contains(mf)&&!superSetDevices.contains(mf)) // filter...
.map(f->XXX.convert2Pojo(f)) // convert to pojo
.forEach(pojoList::add) // add to pojolist, or collect?
Using DCM4CHE to retrieve values based on the tag name in plain xml is pretty straightforward.
For example if I want to retrieve the value of the attribute AccessionNumber:
String accessiongNumber = attribute.getString(Tag.AccessionNumber);
But what is the best approach when dealing with Sequence? I want to retrieve a value using its Tag name, but the value is 5 layers deep inside a Sequence.
In this case, I can get to the Sequence I want with:
Sequence recordSequence = attribute.getSequence(Tag.RecordSequence);
Is there a way to retrieve a value by its tag directly once I have the sequence that the value is embedded in?
Try using the Attributes.getNestedDataset methods. These will get you the attributes in the sequence. Something like:
Attributes refStudy = attribute.getNestedDataset(Tag.ReferencedStudySeequence, 0);
String refSopiuid = refStudy.getString(Tag.ReferencedSOPInstanceUID);
I am using the AWS Java SDK for communicating with DynamoDB. I am trying to do a table update of some properties stored in a map.
Before the update, I have an object that looks like this:
{
"myMap": {
"innerMap": {}
},
"hashKeyName": "hashKeyValue"
}
My code looks like this:
Table myTable = ...;
myTable.updateItem("hashKeyName", "hashKeyValue",
new AttributeUpdate("myMap.innerMap.myKey").addNumeric(100));
After this update, my Dynamo object looks like this (notice that the map is still empty):
{
"myMap": {
"innerMap": {}
},
"myMap.innerMap.myKey": 100,
"hashKeyName": "hashKeyValue"
}
Why was myMap.innerMap.myKey added as a separate field instead of being correctly set in the map?
The reason for this is because using AttributeUpdate is considered a legacy operation and the usage of map keys as attribute names is not supported. The correct usage to use an update expression:
myTable.updateItem("hashKeyName", "hashKeyValue",
"ADD myMap.innerMap.myKey :val", null /* NameMap, see comment */,
new ValueMap().withNumeric(":val", 100));
Notice that there is no name map in the above expression. One might be tempted to do use ADD #name :val as an update expression and then provide a name map for #name => myMap.innerMap.myKey. In this case, these expressions are not equivalent. When a . appears in the raw expression, it is treated as a path separator. When . appears in a NameMap value, it is not considered a path separator.
I have a XML file containing metadata like a field's maximum length. I have to use drools to build rules to validate this metadata against a list of facts. I don't want to hardcode the name of each field that may or may not be specified in the XML.
I tried to do this :
when
$metadata: Metadata(maxLength != null);
$obj: Object(eval($metadata.getFieldName()).length > $metadata.maxLength);
then
// TODO
end
It does not work and I get the following error :
java.lang.IllegalStateException: Errors while building rules : Unable
to Analyse Expression $metadata.getFieldName() > $metadata.maxLength:
[Error: Comparison operation requires compatible types. Found class
java.lang.String and class java.lang.Integer] [Near : {...
$metadata.getFieldName() > $metadata.maxLength ....}]
Is it possible to dynamically get a field name and compare its maximum length? Will I have to create a java object to accomplish this?
Thank you
You talk of XML and metadata. Can you distinguish all entities? For example, if it is about orders, can you extract each order, and attributes of each order?
I solved a similar problem with using maps to store each attribute.
public class Order{
private int id;
private Map<String, Integer> num_attribute_map = new HashMap<String, Integer>();
public Map getNumAttributeMap(){
return this.num_attribute_map;
}
If an order has customer_satisfaction = 5,
order_obj.getNumAttributeMap().put("customer_satisfaction" , 5);
And thus you have created Orders with their attributes stored in the numAttributeMap.
For implementing a rule on an Order
$ord : Order(
getNumAttributeMap[$attribute] >= $value
)
where $attribute would be "customer_satisfaction", of course. The [] notation is used to access elements of a list, given index or values of a map, given the key.
Hope you "get" the concepts of maps. Also, do look up Drools language support for list and map access.
I have also implemented maps of lists of strings to perform an "is in" operation, in addition to maps of integers that do comparison operations. Please refer https://stackoverflow.com/a/9241089/604511 too
Finally, I have decided to generate my drools file dynamically from my XML using rule templates.