Developing Mobile Apps Using DynamoDB - Mastering DynamoDB - page 280

Using local secondary indexes

We have already seen the local secondary indexes in Chapter 2 , Data Models . So, if
someone wants to query a DynamoDB table data using attributes other than the primary
key attributes, then we need to use local secondary index.

Suppose we modify our product table to accommodate one more attribute named
lastModifiedBy , this attribute would be updated every time some user makes changes
to it. Our requirement is to find the change made by a certain user for a given product ID.
To do so, we need to create a local secondary index on lastModifiedBy . Here is how
the table will look now:

productId (hash key) recordId (range key) Data lastModifiedBy (Local Secondary Index)

123

productName

BMW Z Alice

123 cost $20000 Bob

456

productName

Hill range bicycle Alice

456 cost $120 Alice

789

productName

Base ball bat Bob

789 cost $50 Bob

Android

Here is how we have to write queries to fetch an attribute modified by a certain user:

// create map for keys
Map keyConditions = new HashMap();

// Specify the key conditions , here product id = 123
Condition hashKeyCondition = new Condition()
.withComparisonOperator(ComparisonOperator.EQ.toString())
.withAttributeValueList(new
AttributeValue().withN("123"));