CODEWITHSUNDEEP
amazon-web-servicesamazon-dynamodbdynamodb-queries
Amrit Pal Singh
Amrit Pal Singh

Reputation: 7142

Dynamodb Query : get matched objects with filter expression

My data is stored in a table "mysettings" in DynamoDB with the following schema : "pk" as hash key and "sk" as range key

Example data item:

{
  "mSettings": {
    "gflag": true,
    "name": "green",
    "type": "B"
  },
  "pk": "Group1",
  "sk": "A1000",
  "xSettings": {
    "gflag": false,
    "name": "blue",
    "type": "A"
  },
  "ySettings": {
    "gflag": false,
    "name": "green",
    "type": "B"
  },
  "zSettings": {
    "gflag": true,
    "name": "red",
    "type": "A"
  }
}

Now, here I am trying to fetch(filter) only those settings where my gflag field is true. Like for above example item, it should return only "mSettings" & "zSettings" nodes.

Below is my tried sample code: 

      var AWS = require('aws-sdk');
      let region = "us-east-1";
      AWS.config.region = region;
      var docClient = new AWS.DynamoDB.DocumentClient();

      let settingsItem = ["mSettings", "xSettings", "ySettings", "zSettings"];

      let params = {
          TableName: "mysettings",
          KeyConditionExpression: "#pk = :pk AND #sk = :sk",
          ExpressionAttributeNames:{
              "#pk": "pk",
              "#sk": "sk"
          },
         // FilterExpression : "ySettings.gflag = :gflag",
          ExpressionAttributeValues: {
              ":pk": 'Group1',
              ":sk": 'A1000',
              ":gflag" : true
          }     
       };

       //for(let settings in settingsItem){
       //   params['FilterExpression'] = settingsItem[settings] + ".gflag = :gflag";
       //}

      console.log('params = ', params)
      docClient.query(params, function(err, data){
          if(err){
              console.log(err)
          }
          console.log('data = ', data)    
      })

Kindly suggest to retrieve only those objects where flag value is true.

Upvotes: 0

Views: 1074

Answers (1)

jarmod
jarmod

Reputation: 78908

DynamoDB filters operate at an item level. They determine whether or not an item is returned to you. They can't be used to control which groups of attributes are returned to you. You can easily do that client-side with the query results.

Here's an example of post-processing the query results object to remove the sub-objects where gflag is not true:

const _ = require('lodash');

function gflag_true(value, key) {
  return _.isObject(value) && value.gflag;
}

const item = {
  "mSettings": { "gflag": true, "name": "green", "type": "B" },
  "pk": "Group1",
  "sk": "A1000",
  "xSettings": { "gflag": false, "name": "blue", "type": "A" },
  "ySettings": { "gflag": false, "name": "green", "type": "B" },
  "zSettings": { "gflag": true, "name": "red", "type": "A" }
}

// transform item returned from DocumentClient query method
const rc = _.pickBy(item, gflag_true)

This results in:

{
  mSettings: { gflag: true, name: 'green', type: 'B' },
  zSettings: { gflag: true, name: 'red', type: 'A' }
}

Upvotes: 1

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