"""create paper template schema

Creates the three tables behind the template configuration feature:

``section_field``  the reusable heading library (name / level / font size /
                   colour). Deliberately flat — there is no ``parent_id``, so a
                   level-2 heading can be reused under any number of parents
                   and inside any number of templates.
``paper_template`` a named outline: name + abstract.
``template_field`` the join table, and the only place a display position is
                   recorded (``sort``). It has **no** unique constraint on
                   ``(template_id, field_id)``: placing the same field twice in
                   one template is a legitimate layout, and the UI warns about
                   it rather than the schema forbidding it.

Note on the foreign keys below: TiDB parses ``FOREIGN KEY`` for compatibility
but does not enforce it. They are declared to document the relationships, and
the integrity they would provide is enforced in the application layer instead.

Revision ID: 5030e3939a26
Revises:
Create Date: 2026-09-18 15:46:10.533951

"""

from collections.abc import Sequence

import sqlalchemy as sa
from alembic import op

# revision identifiers, used by Alembic.
revision: str = "5030e3939a26"
down_revision: str | None = None
branch_labels: str | Sequence[str] | None = None
depends_on: str | Sequence[str] | None = None

#: ``CURRENT_TIMESTAMP`` rather than ``now()``: it is the spelling MySQL and
#: TiDB both accept as a DATETIME column default without the expression-default
#: parentheses that only newer MySQL versions allow.
_NOW = sa.text("CURRENT_TIMESTAMP")


def upgrade() -> None:
    op.create_table(
        "paper_template",
        sa.Column("id", sa.Integer(), autoincrement=True, nullable=False),
        sa.Column("name", sa.String(length=255), nullable=False),
        sa.Column("abstract", sa.Text(), nullable=True),
        sa.Column("created_at", sa.DateTime(), server_default=_NOW, nullable=False),
        sa.Column("updated_at", sa.DateTime(), server_default=_NOW, nullable=False),
        sa.PrimaryKeyConstraint("id"),
    )

    op.create_table(
        "section_field",
        sa.Column("id", sa.Integer(), autoincrement=True, nullable=False),
        # Name carries the user's own numbering ("1. Introduction"); the API
        # stores and returns it verbatim.
        sa.Column("name", sa.String(length=255), nullable=False),
        # Heading depth as a rendering hint: 1 = "1.", 2 = "1.1".
        sa.Column("level", sa.Integer(), server_default=sa.text("1"), nullable=False),
        # DECIMAL(4,1) because conventional Chinese sizes are fractional
        # (五号 = 10.5pt).
        sa.Column(
            "font_size",
            sa.Numeric(precision=4, scale=1),
            server_default=sa.text("12.0"),
            nullable=False,
        ),
        # Canonical RGB as #RRGGBB.
        sa.Column(
            "font_color",
            sa.String(length=7),
            server_default=sa.text("'#000000'"),
            nullable=False,
        ),
        sa.Column("created_at", sa.DateTime(), server_default=_NOW, nullable=False),
        sa.Column("updated_at", sa.DateTime(), server_default=_NOW, nullable=False),
        sa.PrimaryKeyConstraint("id"),
    )

    op.create_table(
        "template_field",
        sa.Column("id", sa.Integer(), autoincrement=True, nullable=False),
        sa.Column("template_id", sa.Integer(), nullable=False),
        sa.Column("field_id", sa.Integer(), nullable=False),
        # Ascending integer; the sole determinant of render order.
        sa.Column("sort", sa.Integer(), nullable=False),
        sa.ForeignKeyConstraint(
            ["field_id"], ["section_field.id"], ondelete="RESTRICT"
        ),
        sa.ForeignKeyConstraint(
            ["template_id"], ["paper_template.id"], ondelete="CASCADE"
        ),
        sa.PrimaryKeyConstraint("id"),
    )
    # Every read is "the fields of template X, in order", so one composite
    # index serves both the filter and the sort.
    op.create_index(
        "ix_template_field_template_sort",
        "template_field",
        ["template_id", "sort"],
        unique=False,
    )
    op.create_index(
        "ix_template_field_field_id", "template_field", ["field_id"], unique=False
    )


def downgrade() -> None:
    # Reverse dependency order: the join table goes before the tables it points
    # at.
    op.drop_index("ix_template_field_field_id", table_name="template_field")
    op.drop_index("ix_template_field_template_sort", table_name="template_field")
    op.drop_table("template_field")
    op.drop_table("section_field")
    op.drop_table("paper_template")