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How to dynamically load evaluation parameters from a database

This guide will help you create an Expectation that loads part of its Expectation Configuration from a database at runtime. Using a dynamic Evaluation Parameter makes it possible to maintain part of an Expectation Suite in a shared database.

Prerequisites: This how-to guide assumes you have:


  1. Add a new SqlAlchemy Query Store to your context

    A SqlAlchemy Query Store acts as a bridge that can query a SqlAlchemy-connected database and return the result of the query to be available for an evaluation parameter.

    Find the stores section in your great_expectations.yml file, and add the following configuration for a new store called "my_query_store". You can add and reference multiple Query Stores with different names.

    my_query_store:class_name: SqlAlchemyQueryStorecredentials: ${rds_movies_db}queries:  current_genre_ids: "SELECT id FROM genres;"  # The query name and value can be replaced with your desired query

    By default, query results will be returned as a list. If instead you need a scalar for your expectation, you can specify the return_type

    my_query_store:class_name: SqlAlchemyQueryStorecredentials: ${rds_movies_db}queries:  current_ratings_max:    query: "SELECT MAX(rating) FROM ratings;"    return_type: "scalar"  # return_type can be either "scalar" or "list" or omitted  current_genre_ids:    query: "SELECT id FROM genres;"    return_type: "list"  # return_type can be either "scalar" or "list" or omitted

    Ensure you have added valid credentials to the config-variables.yml file:

    rds_movies_db:  drivername: postgresql  host: <<hostname>>  # Update with your hostname  port: 5432  username: testuser  # Update with your username  password: <<password>>  # Update with your password  database: testdb  # Update with your database
  2. In a notebook, get a test batch of data to use for validation.

    import great_expectations as gecontext = ge.DataContext()
    batch_kwargs = {    "datasource": "movies_db",    "table": "genres_movies"}expectation_suite_name = "genres_movies.fkey"context.create_expectation_suite(expectation_suite_name, overwrite_existing=True)batch = context.get_batch(    batch_kwargs=batch_kwargs,    expectation_suite_name=expectation_suite_name)
  1. Define an expectation that relies on a dynamic query

    Great Expectations recognizes several types of Evaluation Parameters that can use advanced features provided by the Data Context. To dynamically load data, we will be using a store-style URN, which starts with "urn:great_expectations:stores". The next component of the URN is the name of the store we configured above (my_query_store), and the final component is the name of the query we defined above (current_genre_ids):

    batch.expect_column_values_to_be_in_set(    column="genre_id",    value_set={"$PARAMETER": "urn:great_expectations:stores:my_query_store:current_genre_ids"})

    The SqlAlchemyQueryStore that you configured above will execute the defined query and return the results as the value of the value_set parameter to evaluate your expectation:

    {  "meta": {    "substituted_parameters": {      "value_set": [        1,        2,        3,        4,        5,        6,        7,        8,        9,        10,        11,        12,        13,        14,        15,        16,        17,        18      ]    }  },  "result": {    "element_count": 2891,    "missing_count": 0,    "missing_percent": 0.0,    "unexpected_count": 0,    "unexpected_percent": 0.0,    "unexpected_percent_nonmissing": 0.0,    "partial_unexpected_list": []  },  "success": true,  "exception_info": null}