-- Registry of flexible field definitions (the "schema of schemas").
CREATE TABLE field_definition (
    id             UUID PRIMARY KEY,
    key            TEXT NOT NULL UNIQUE CHECK (key <> ''),
    data_type      TEXT NOT NULL CHECK (data_type IN
                       ('text', 'localized_text', 'integer', 'date', 'boolean', 'term', 'authority')),
    vocabulary_id  UUID REFERENCES vocabulary (id) ON DELETE RESTRICT,
    authority_kind TEXT CHECK (authority_kind IN ('person', 'organisation', 'place')),
    required       BOOLEAN NOT NULL DEFAULT false,
    group_key      TEXT CHECK (group_key <> ''),
    -- A term field must name a vocabulary; any other type must not.
    CONSTRAINT term_has_vocabulary CHECK ((data_type = 'term') = (vocabulary_id IS NOT NULL)),
    -- authority_kind is only meaningful for authority fields.
    CONSTRAINT authority_kind_only_for_authority
        CHECK (authority_kind IS NULL OR data_type = 'authority')
);

CREATE TABLE field_definition_label (
    field_definition_id UUID NOT NULL REFERENCES field_definition (id) ON DELETE CASCADE,
    lang                TEXT NOT NULL CHECK (lang <> ''),
    label               TEXT NOT NULL CHECK (label <> ''),
    PRIMARY KEY (field_definition_id, lang)
);