"""Keep immutable document revisions for reindex and historical citations.

Revision ID: 0008
Revises: 0007
"""
from alembic import op
import sqlalchemy as sa

revision = "0008"
down_revision = "0007"
branch_labels = None
depends_on = None

SOURCE_COLUMNS = ["course_version_id", "project_id", "submission_file_id", "source_hash"]
REVISION_CONSTRAINT = "uq_knowledge_document_revision"
NAMING = {"uq": "uq_%(table_name)s_%(column_0_name)s"}


def upgrade():
    inspector = sa.inspect(op.get_bind())
    columns = {column["name"] for column in inspector.get_columns("knowledge_documents")}
    constraints = inspector.get_unique_constraints("knowledge_documents")
    # Migration 0001 uses current metadata for fresh installs; handle both fresh
    # schema and a deployed 0007 schema without a revision column.
    with op.batch_alter_table("knowledge_documents", naming_convention=NAMING) as batch:
        if "revision" not in columns:
            batch.add_column(sa.Column("revision", sa.Integer(), nullable=False, server_default="1"))
        for constraint in constraints:
            if constraint["column_names"] == SOURCE_COLUMNS:
                batch.drop_constraint(constraint["name"] or "uq_knowledge_documents_course_version_id", type_="unique")
        if not any(c["column_names"] == SOURCE_COLUMNS + ["revision"] for c in constraints):
            batch.create_unique_constraint(REVISION_CONSTRAINT, SOURCE_COLUMNS + ["revision"])


def downgrade():
    # Restoring the old uniqueness would require deleting citation sources. Stop
    # before schema changes rather than silently choosing a surviving revision.
    duplicates = op.get_bind().execute(sa.text(
        "SELECT 1 FROM knowledge_documents GROUP BY course_version_id, project_id, "
        "submission_file_id, source_hash HAVING COUNT(*) > 1 LIMIT 1")).first()
    if duplicates:
        raise RuntimeError("存在同源文档的历史修订，不能无损降级；请保留 0008 或恢复完整升级前备份")
    with op.batch_alter_table("knowledge_documents", naming_convention=NAMING) as batch:
        batch.drop_constraint(REVISION_CONSTRAINT, type_="unique")
        batch.create_unique_constraint("uq_knowledge_documents_source", SOURCE_COLUMNS)
        batch.drop_column("revision")