from odoo import _, api, fields, models
from odoo.exceptions import ValidationError


class McEnrollment(models.Model):
    _name = "mc.enrollment"
    _inherit = ["mail.thread"]
    _description = "Enrollment"
    _order = "year_id desc, student_id"

    student_id = fields.Many2one(
        "mc.student", string="Student", required=True, ondelete="restrict",
    )
    program_id = fields.Many2one(
        "mc.program", string="Program", required=True, ondelete="restrict",
    )
    batch_id = fields.Many2one(
        "mc.batch", string="Batch", required=True, ondelete="restrict",
    )
    year_id = fields.Many2one(
        "mc.academic.year", string="Academic Year", required=True, ondelete="restrict",
    )
    state = fields.Selection(
        [
            ("draft", "Draft"),
            ("active", "Active"),
            ("completed", "Completed"),
            ("withdrawn", "Withdrawn"),
            ("transferred", "Transferred"),
        ],
        string="Status", default="draft", required=True, tracking=True,
    )
    roll_no = fields.Char(string="Roll No.", tracking=True)
    date_enrolled = fields.Date(string="Date Enrolled", default=fields.Date.context_today)

    @api.depends("student_id.name", "year_id.name", "batch_id.name")
    def _compute_display_name(self):
        for enrollment in self:
            enrollment.display_name = "%s - %s (%s)" % (
                enrollment.student_id.name or "?",
                enrollment.batch_id.name or "?",
                enrollment.year_id.name or "?",
            )

    def init(self):
        # The spine of the whole domain model: a student has at most one
        # Active enrollment per academic year. This MUST be a database
        # constraint, not application logic - CLAUDE.md is explicit about
        # this one. This partial unique index is the actual guarantee.
        # Because it fires synchronously at INSERT/UPDATE time, it runs
        # BEFORE @api.constrains ever gets a chance to - a naive
        # "index + constrains for a friendly message" design is not
        # enough, the raw IntegrityError wins that race. create()/write()
        # below pre-check and raise the friendly message first for the
        # common single-record case; @api.constrains stays as a backstop
        # for batch creates where sibling rows in the same vals_list
        # can't see each other yet at pre-check time.
        self.env.cr.execute(
            "CREATE UNIQUE INDEX IF NOT EXISTS mc_enrollment_one_active_per_student_per_year "
            "ON mc_enrollment (student_id, year_id) WHERE state = 'active'"
        )

    def _check_no_conflicting_active_enrollment(self, student_id, year_id, exclude_id=None):
        domain = [
            ("student_id", "=", student_id),
            ("year_id", "=", year_id),
            ("state", "=", "active"),
        ]
        if exclude_id:
            domain.append(("id", "!=", exclude_id))
        duplicate = self.search(domain, limit=1)
        if duplicate:
            raise ValidationError(_(
                "%(student)s already has an active enrollment for %(year)s.",
                student=duplicate.student_id.name,
                year=duplicate.year_id.name,
            ))

    @api.constrains("student_id", "year_id", "state")
    def _check_one_active_enrollment_per_year(self):
        for enrollment in self:
            if enrollment.state == "active":
                self._check_no_conflicting_active_enrollment(
                    enrollment.student_id.id, enrollment.year_id.id,
                    exclude_id=enrollment.id,
                )

    @api.model_create_multi
    def create(self, vals_list):
        for vals in vals_list:
            if vals.get("state") == "active":
                self._check_no_conflicting_active_enrollment(
                    vals.get("student_id"), vals.get("year_id"),
                )
        # A state change on another enrollment earlier in the same
        # transaction (e.g. withdrawing the old one before enrolling the
        # new one) sits in the ORM cache until flushed. create() issues a
        # direct SQL INSERT that will not wait for it, so without this
        # flush the partial unique index can still see the old row as
        # active and reject a perfectly legitimate re-enrollment.
        self.env.flush_all()
        return super().create(vals_list)

    def write(self, vals):
        if vals.get("state") == "active":
            for enrollment in self:
                self._check_no_conflicting_active_enrollment(
                    vals.get("student_id", enrollment.student_id.id),
                    vals.get("year_id", enrollment.year_id.id),
                    exclude_id=enrollment.id,
                )
        return super().write(vals)