budget-tracker/backend/app/services/dashboard_service.py

import uuid
from datetime import date

from sqlalchemy import func, select
from sqlalchemy.ext.asyncio import AsyncSession
from sqlalchemy.orm import selectinload

from app.models.budget import Budget
from app.models.category import Category
from app.models.transaction import Transaction, TransactionType
from app.schemas.dashboard import (
    BudgetAlert,
    CategoryExpense,
    DashboardResponse,
    MonthlyTrend,
)


def _month_bounds(month: str) -> tuple[date, date]:
    year, mon = map(int, month.split("-"))
    start = date(year, mon, 1)
    if mon == 12:
        end = date(year + 1, 1, 1)
    else:
        end = date(year, mon + 1, 1)
    return start, end


def _prev_month(month: str) -> str:
    year, mon = map(int, month.split("-"))
    if mon == 1:
        return f"{year - 1}-12"
    return f"{year}-{mon - 1:02d}"


async def get_dashboard(
    session: AsyncSession,
    user_id: uuid.UUID,
    month: str,
) -> DashboardResponse:
    start, end = _month_bounds(month)

    # --- All-time balance (cumulative since origin) ---
    balance_result = await session.execute(
        select(
            func.coalesce(
                func.sum(
                    func.case(
                        (Transaction.type == TransactionType.income, Transaction.amount_cents),
                        else_=-Transaction.amount_cents,
                    )
                ),
                0,
            )
        ).where(
            Transaction.user_id == user_id,
            Transaction.deleted_at.is_(None),
        )
    )
    balance_cents: int = balance_result.scalar_one()

    # --- Month income & expenses ---
    month_result = await session.execute(
        select(
            func.coalesce(
                func.sum(
                    func.case(
                        (Transaction.type == TransactionType.income, Transaction.amount_cents),
                        else_=0,
                    )
                ),
                0,
            ).label("income"),
            func.coalesce(
                func.sum(
                    func.case(
                        (Transaction.type == TransactionType.expense, Transaction.amount_cents),
                        else_=0,
                    )
                ),
                0,
            ).label("expense"),
        ).where(
            Transaction.user_id == user_id,
            Transaction.deleted_at.is_(None),
            Transaction.transaction_date >= start,
            Transaction.transaction_date < end,
        )
    )
    month_row = month_result.one()
    total_income_cents: int = month_row.income
    total_expense_cents: int = month_row.expense

    # --- Expenses by category for the month ---
    cat_result = await session.execute(
        select(
            Transaction.category_id,
            func.sum(Transaction.amount_cents).label("total"),
        )
        .where(
            Transaction.user_id == user_id,
            Transaction.deleted_at.is_(None),
            Transaction.type == TransactionType.expense,
            Transaction.transaction_date >= start,
            Transaction.transaction_date < end,
        )
        .group_by(Transaction.category_id)
    )
    cat_rows = cat_result.all()

    # Fetch category names
    category_ids = [row.category_id for row in cat_rows]
    by_category: list[CategoryExpense] = []
    if category_ids:
        cats_result = await session.execute(
            select(Category).where(Category.id.in_(category_ids))
        )
        cats_map = {c.id: c for c in cats_result.scalars().all()}
        for row in cat_rows:
            cat = cats_map.get(row.category_id)
            by_category.append(
                CategoryExpense(
                    category_id=row.category_id,
                    category_name=cat.name if cat else "?",
                    color=cat.color if cat else None,
                    amount_cents=row.total,
                )
            )

    # --- Monthly trend: last 6 months ---
    monthly_trend: list[MonthlyTrend] = []
    cur = month
    months_to_fetch = []
    for _ in range(6):
        months_to_fetch.append(cur)
        cur = _prev_month(cur)
    months_to_fetch.reverse()

    for m in months_to_fetch:
        m_start, m_end = _month_bounds(m)
        trend_result = await session.execute(
            select(
                func.coalesce(
                    func.sum(
                        func.case(
                            (Transaction.type == TransactionType.income, Transaction.amount_cents),
                            else_=0,
                        )
                    ),
                    0,
                ).label("income"),
                func.coalesce(
                    func.sum(
                        func.case(
                            (Transaction.type == TransactionType.expense, Transaction.amount_cents),
                            else_=0,
                        )
                    ),
                    0,
                ).label("expense"),
            ).where(
                Transaction.user_id == user_id,
                Transaction.deleted_at.is_(None),
                Transaction.transaction_date >= m_start,
                Transaction.transaction_date < m_end,
            )
        )
        trend_row = trend_result.one()
        monthly_trend.append(
            MonthlyTrend(
                month=m,
                income_cents=trend_row.income,
                expense_cents=trend_row.expense,
            )
        )

    # --- Budget alerts (>= 80% spent) ---
    budgets_result = await session.execute(
        select(Budget)
        .options(selectinload(Budget.category))
        .where(
            Budget.user_id == user_id,
            Budget.month == month,
        )
    )
    budgets = budgets_result.scalars().all()

    budget_alerts: list[BudgetAlert] = []
    for budget in budgets:
        spent_result = await session.execute(
            select(func.coalesce(func.sum(Transaction.amount_cents), 0)).where(
                Transaction.user_id == user_id,
                Transaction.deleted_at.is_(None),
                Transaction.type == TransactionType.expense,
                Transaction.category_id == budget.category_id,
                Transaction.transaction_date >= start,
                Transaction.transaction_date < end,
            )
        )
        spent_cents: int = spent_result.scalar_one()
        percentage = (spent_cents / budget.limit_cents * 100) if budget.limit_cents > 0 else 0
        if percentage >= 80:
            budget_alerts.append(
                BudgetAlert(
                    budget_id=budget.id,
                    category_name=budget.category.name,
                    limit_cents=budget.limit_cents,
                    spent_cents=spent_cents,
                    percentage=round(percentage, 1),
                )
            )

    return DashboardResponse(
        month=month,
        balance_cents=balance_cents,
        total_income_cents=total_income_cents,
        total_expense_cents=total_expense_cents,
        net_cents=total_income_cents - total_expense_cents,
        by_category=by_category,
        monthly_trend=monthly_trend,
        budget_alerts=budget_alerts,
    )