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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!

bronzeson
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  • *UpdateExpression with variables and not a hard-coded String*: What should not be hard-coded? The key (I guess `name` in your case), or the attributes values to be set? – vahdet Mar 15 '19 at 17:23
  • None of them should be hard-coded. I am looping over different objects that will all have different names based on their contents – bronzeson Mar 15 '19 at 17:26
  • I'm not familiar at all with Java but I think this example uses dynamically defined key and values https://docs.aws.amazon.com/sdk-for-java/v1/developer-guide/examples-dynamodb-items.html. – cementblocks Mar 15 '19 at 18:37

1 Answers1

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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?

readyornot
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