Summary: json_populate_record(base_record, json) maps a JSON object onto a pre-defined PostgreSQL composite type or table row, returning a single typed record with JSON values merged in. Use it instead of json_to_record when the row type is already defined and you need missing JSON keys to fall back to values from the base record rather than NULL. For JSONB input, use jsonb_populate_record; for multiple rows from a JSON array, use json_populate_recordset.

Casts a JSON object to a record

The json_populate_record function is used to populate a record type with values from a JSON object. It is useful for parsing JSON data received from external sources, particularly when merging it into an existing record.

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json_populate_record(base_record ANYELEMENT, json JSON)

This function takes two arguments: a base record of a row type (which can even be a NULL record) and a JSON object. It returns the record updated with the JSON values.

Consider a database table that tracks employee information. When you receive employee information as JSON records, you can use json_populate_record to ingest the data into the table.

Here we create the employees table with some sample data.

CREATE TABLE employees (
    id INT,
    name TEXT,
    department TEXT,
    salary NUMERIC
);

To illustrate, we start with a NULL record and cast the input JSON payload to the employees record type.

INSERT INTO employees
SELECT *
FROM json_populate_record(
    NULL::employees,
    '{"id": "123", "name": "John Doe", "department": "Engineering", "salary": "75000"}'
)
RETURNING *;

This query returns the following result:

| id | name     | department  | salary |
|----|----------|-------------|--------|
| 123| John Doe | Engineering | 75000  |

For data points where the JSON objects have missing keys, json_populate_record can still cast them into legible records.

Say we receive records for a bunch of employees who are known to be in Sales, but the department field is missing from the JSON payload. We can use json_populate_record with the default value specified for a field while the other fields are populated from the JSON payload, as in this example:

INSERT INTO employees
SELECT *
FROM json_populate_record(
    (1, 'ABC', 'Sales', 0)::employees,
    '{"id": "124", "name": "Jane Smith", "salary": "68000"}'
)
RETURNING *;

This query returns the following:

| id | name       | department | salary |
|----|------------|------------|--------|
| 124| Jane Smith | Sales      | 68000  |

The base record doesn't need to have the type of a table row and can be a custom Postgres type too. For example, here we first define a custom type address and use json_populate_record to cast a JSON object to it:

CREATE TYPE address AS (
    street TEXT,
    city TEXT,
    zip TEXT
);

SELECT *
FROM json_populate_record(
    NULL::address,
    '{"street": "123 Main St", "city": "San Francisco", "zip": "94105"}'
);

This query returns the following result:

| street     | city          | zip   |
|------------|---------------|-------|
| 123 Main St| San Francisco | 94105 |

• json_to_record - It can be used similarly, with a couple differences. json_populate_record can be used with a base record of a pre-defined type, whereas json_to_record needs the record type defined inline in the AS clause. Further, json_populate_record can specify default values for missing fields through the base record, whereas json_to_record must assign them NULL values.
• json_populate_recordset - It can be used similarly to parse JSON, the difference being that it returns a set of records instead of a single record. For example, if you have an array of JSON objects, you can use json_populate_recordset to convert each object into a new row.
• jsonb_populate_record - It has the same functionality to json_populate_record, but accepts JSONB input instead of JSON.

• Postgres documentation: JSON functions

• array_to_json
• json
• json_agg
• json_array_elements
• json_build_object
• json_each
• json_exists
• json_extract_path
• json_extract_path_text
• json_object
• json_query
• json_scalar
• json_serialize
• json_table
• json_to_record
• json_value
• jsonb_array_elements
• jsonb_each
• jsonb_extract_path
• jsonb_extract_path_text
• jsonb_object
• jsonb_populate_record
• jsonb_to_record

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