DBT: Relationships test in DBT between two source tables

Question

Currently i have a 2 table :

PRODUCTS (which is having PRD_ID, PRD_NAME, PRD_VERSION,PRD_CATEGORY, PRD_PRICE columns) TRANSACTIONS (which is having TR_ID,TR_DATE,TR_LINE_ID,TR_PRD_ID,TR_PRD_QTY column

PRD_ID Is primary key in Products table and TR_ID is primary key in Transactions table. TR_PRD_ID is foreign key representing PRD_ID column from Products table.

I have set up dbt tests in schema.yml file as below:

version: 2


sources:
  - name: sources_demo
    database: hands_on_demo
    schema: seeds_demo
    tables: 
      - name: src_transactions
        identifier: TRANSACTIONS
        description: "transactions details"
        columns:
            - name: TR_ID
              tests:
                - unique
                - not_null
            - name: TR_LINE_ID
                - not_null
            - name: TR_PRD_ID
                - not_null
                - relationships:
                      to: source('sources_demo','src_products')
                      field: PRD_ID   
            - name: TR_PRD_QTY
                - not_null
        tests:
          - unique:
              column_name: "TR_ID ||'-'|| TR_LINE_ID"

      - name: src_products
        identifier: PRODUCTS
        description: "All Products details"
        columns:
            - name: PRD_ID
              tests:
                - unique
                - not_null
            - name: PRD_PRICE
              tests:
                - not_null
            - name: PRD_CATEGORY
              tests:
                - not_null
                - accepted_values:
                      values: ['Charger', 'Mobile', 'SmartWatch']

Every time i am running i am getting below error :

11:47:59  Registered adapter: snowflake=1.7.1
11:47:59  Encountered an error:
Parsing Error
  Error reading dbthandson: sources.yml - Runtime Error
    Syntax error near line 20
    ------------------------------
    17 |                 - not_null
    18 |             - name: TR_PRD_ID
    19 |                 - not_null
    20 |                 - relationships:
    21 |                       to: source('sources_demo','src_products')
    22 |                       field: PRD_ID   
    23 |             - name: TR_PRD_QTY
    
    Raw Error:
    ------------------------------
    mapping values are not allowed in this context
      in "<unicode string>", line 20, column 32

what is the issue here ?? What am i doing wrong ?