Summary

Implements support for singleton tables — tables with empty primary keys that can hold at most one row. This feature was described in the 2018 DataJoint paper and proposed in #113.

Syntax

@schema
class Config(dj.Lookup):
    definition = """
    # Global configuration
    ---
    setting1 : varchar(100)
    setting2 : int32
    """

A definition with no attributes before --- creates a singleton table.

Implementation

• Uses a hidden _singleton attribute of type bool as the primary key
• The attribute is automatically created and populated with DEFAULT TRUE
• Excluded from heading.attributes, fetch() results, and join matching
• Visible heading.primary_key returns [] (empty)
• Works with both MySQL and PostgreSQL (adapter-specific boolean literals)

Changed Files

• src/datajoint/declare.py - Singleton table creation logic
• src/datajoint/adapters/base.py - Add boolean_true_literal property
• src/datajoint/adapters/postgres.py - PostgreSQL-specific boolean literal
• tests/integration/test_declare.py - Comprehensive test coverage

Behavior

Use Cases

• Global configuration settings
• Pipeline parameters
• Summary statistics
• State tracking (e.g., current term in a university database)

Test Plan

• test_singleton_declaration - Table creates with hidden _singleton attribute
• test_singleton_insert_and_fetch - Insert/fetch work without specifying key
• test_singleton_uniqueness - Second insert raises DuplicateError
• test_singleton_with_multiple_attributes - Works with multiple secondary attrs
• test_singleton_describe - Describe shows singleton nature

Closes #113

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