Query dynamodb for a specific field

Question

how do I query dynamodb for all items that match a specific rule?

I am building an online shop (like Asos) and in dynamodb I have a products table. all of the products have the field 'category'. so I just want to do a simple query whereby I say something like

FROM products WHERE category == 'trainers'

I have managed to get it working so I scan the whole table and return me all the products but i want more specific queries now

I assume I want to use something like: KeyConditionExpression but I keep getting an error when I do this

edited question:

code:

componentDidMount() {
    console.log('mounted');
    const AWS = window.AWS;
    const params = {
      TableName: 'Products',
      FilterExpression: "category = trainers"
    };
...secret key code
ddb.scan(params, (err, data) => {
  if (err) {
    console.log(err, err.stack);
  } else {
    console.log(data);
    this.setState({ products: data })
  }
});

If you post your schema, the query you are currently trying and what you want to get, I can help. — F_SO_K, Jan 21 '18 at 10:00
@Stu I have one table and I just want to pull stuff from it based on a field/column called category. and if there are 10 products, I only want the ones with the category == trainers to be returned. need any more? — The Walrus, Jan 21 '18 at 15:00
You need to know your table schema in order to construct the query. As I say, if you can provide your schema and example code we can go from there. Otherwise there's nothing I can really add that the AWS documents don't already tell you. — F_SO_K, Jan 21 '18 at 15:06
@Stu ok added, that is my table and some code so far where im trying to pull out just trainers so basically products 101 and 102 should be returned — The Walrus, Jan 21 '18 at 15:12
OK, are you getting an error with this code? Whats the error message? — F_SO_K, Jan 21 '18 at 15:15
no error, my console.log just shows > `{Items: Array(0), Count: 0, ScannedCount: 3}` basically saying it scanned the 3 items and didn't find any matches I guess. have I done the filter correctly ? — The Walrus, Jan 21 '18 at 15:18

F_SO_K · Accepted Answer · 2018-01-21T15:29:24.947

2

Try this

 var params = {
  ExpressionAttributeValues: {
   ":catval": {
     S: "trainers"
    }
  }, 
  FilterExpression: "category = :catval", 
  TableName: "Products"
 };
 dynamodb.scan(params, function(err, data) {
  if (err) console.log(err, err.stack);
  else     console.log(data);

EDIT: You can't use literals in expressions. You define an ExpressionAttributeValue, i've called yours ":catval" but you can call it anything you like. The 'S' means a string, so basically you define an ExpressionAttributeValue with type String and value "trainers". Then when you apply the FilterExpression you put in your ExpressionAttributeValue, not the literal.

edited Jan 21 '18 at 15:29

answered Jan 21 '18 at 15:22

F_SO_K

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perfect! can you explain the `:` before catval? everything else makes sense – The Walrus Jan 21 '18 at 15:26
Added in answer – F_SO_K Jan 21 '18 at 15:29
great, much appreciated! – The Walrus Jan 21 '18 at 15:30
No problem. Good luck. Raise another question if you have any, I keep an eye on the dynamodb tag. – F_SO_K Jan 21 '18 at 15:30
cheers, might have a question later regarding how to pull images into DDB from S3 but I'll see how I get on first :). thanks again – The Walrus Jan 21 '18 at 15:36

score 1 · Answer 2 · answered Jan 21 '18 at 12:07

KeyConditionExpression is for when you want data filtered by primary key attributes. If that is not your case, you should use FilterExpression. A filter expression determines which items within the Scan results should be returned to you. All of the other results are discarded.

Query dynamodb for a specific field

2 Answers2

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