from __future__ import unicode_literals

from django.apps import apps
from django.core.management.base import CommandError
from django.db import models, router
from django.utils.version import get_docs_version


def check_for_migrations(app_config, connection):
    # Inner import, else tests imports it too early as it needs settings
    from django.db.migrations.loader import MigrationLoader
    loader = MigrationLoader(connection)
    if app_config.label in loader.migrated_apps:
        raise CommandError(
            "App '%s' has migrations. Only the sqlmigrate and sqlflush commands "
            "can be used when an app has migrations." % app_config.label
        )


def sql_create(app_config, style, connection):
    "Returns a list of the CREATE TABLE SQL statements for the given app."

    check_for_migrations(app_config, connection)

    if connection.settings_dict['ENGINE'] == 'django.db.backends.dummy':
        # This must be the "dummy" database backend, which means the user
        # hasn't set ENGINE for the database.
        raise CommandError(
            "Django doesn't know which syntax to use for your SQL statements,\n"
            "because you haven't properly specified the ENGINE setting for the database.\n"
            "see: https://docs.djangoproject.com/en/%s/ref/settings/#databases" % get_docs_version()
        )

    # Get installed models, so we generate REFERENCES right.
    # We trim models from the current app so that the sqlreset command does not
    # generate invalid SQL (leaving models out of known_models is harmless, so
    # we can be conservative).
    app_models = list(app_config.get_models(include_auto_created=True))
    final_output = []
    tables = connection.introspection.table_names()
    known_models = set(model for model in connection.introspection.installed_models(tables) if model not in app_models)
    pending_references = {}

    for model in router.get_migratable_models(app_config, connection.alias, include_auto_created=True):
        output, references = connection.creation.sql_create_model(model, style, known_models)
        final_output.extend(output)
        for refto, refs in references.items():
            pending_references.setdefault(refto, []).extend(refs)
            if refto in known_models:
                final_output.extend(connection.creation.sql_for_pending_references(refto, style, pending_references))
        final_output.extend(connection.creation.sql_for_pending_references(model, style, pending_references))
        # Keep track of the fact that we've created the table for this model.
        known_models.add(model)

    # Handle references to tables that are from other apps
    # but don't exist physically.
    not_installed_models = set(pending_references.keys())
    if not_installed_models:
        alter_sql = []
        for model in not_installed_models:
            alter_sql.extend('-- ' + sql for sql in
                connection.creation.sql_for_pending_references(model, style, pending_references))
        if alter_sql:
            final_output.append('-- The following references should be added but depend on non-existent tables:')
            final_output.extend(alter_sql)

    return final_output


def sql_delete(app_config, style, connection, close_connection=True):
    "Returns a list of the DROP TABLE SQL statements for the given app."

    check_for_migrations(app_config, connection)

    # This should work even if a connection isn't available
    try:
        cursor = connection.cursor()
    except Exception:
        cursor = None

    try:
        # Figure out which tables already exist
        if cursor:
            table_names = connection.introspection.table_names(cursor)
        else:
            table_names = []

        output = []

        # Output DROP TABLE statements for standard application tables.
        to_delete = set()

        references_to_delete = {}
        app_models = router.get_migratable_models(app_config, connection.alias, include_auto_created=True)
        for model in app_models:
            if cursor and connection.introspection.table_name_converter(model._meta.db_table) in table_names:
                # The table exists, so it needs to be dropped
                opts = model._meta
                for f in opts.local_fields:
                    if f.rel and f.rel.to not in to_delete:
                        references_to_delete.setdefault(f.rel.to, []).append((model, f))

                to_delete.add(model)

        for model in app_models:
            if connection.introspection.table_name_converter(model._meta.db_table) in table_names:
                output.extend(connection.creation.sql_destroy_model(model, references_to_delete, style))
    finally:
        # Close database connection explicitly, in case this output is being piped
        # directly into a database client, to avoid locking issues.
        if cursor and close_connection:
            cursor.close()
            connection.close()

    if not output:
        output.append('-- App creates no tables in the database. Nothing to do.')
    return output[::-1]  # Reverse it, to deal with table dependencies.


def sql_flush(style, connection, only_django=False, reset_sequences=True, allow_cascade=False):
    """
    Returns a list of the SQL statements used to flush the database.

    If only_django is True, then only table names that have associated Django
    models and are in INSTALLED_APPS will be included.
    """
    if only_django:
        tables = connection.introspection.django_table_names(only_existing=True, include_views=False)
    else:
        tables = connection.introspection.table_names(include_views=False)
    seqs = connection.introspection.sequence_list() if reset_sequences else ()
    statements = connection.ops.sql_flush(style, tables, seqs, allow_cascade)
    return statements


def sql_indexes(app_config, style, connection):
    "Returns a list of the CREATE INDEX SQL statements for all models in the given app."

    check_for_migrations(app_config, connection)

    output = []
    for model in router.get_migratable_models(app_config, connection.alias, include_auto_created=True):
        output.extend(connection.creation.sql_indexes_for_model(model, style))
    return output


def sql_destroy_indexes(app_config, style, connection):
    "Returns a list of the DROP INDEX SQL statements for all models in the given app."

    check_for_migrations(app_config, connection)

    output = []
    for model in router.get_migratable_models(app_config, connection.alias, include_auto_created=True):
        output.extend(connection.creation.sql_destroy_indexes_for_model(model, style))
    return output


def sql_all(app_config, style, connection):

    check_for_migrations(app_config, connection)

    "Returns a list of CREATE TABLE SQL, initial-data inserts, and CREATE INDEX SQL for the given module."
    return (
        sql_create(app_config, style, connection) +
        sql_indexes(app_config, style, connection)
    )


def emit_pre_migrate_signal(verbosity, interactive, db):
    # Emit the pre_migrate signal for every application.
    for app_config in apps.get_app_configs():
        if app_config.models_module is None:
            continue
        if verbosity >= 2:
            print("Running pre-migrate handlers for application %s" % app_config.label)
        models.signals.pre_migrate.send(
            sender=app_config,
            app_config=app_config,
            verbosity=verbosity,
            interactive=interactive,
            using=db)


def emit_post_migrate_signal(verbosity, interactive, db):
    # Emit the post_migrate signal for every application.
    for app_config in apps.get_app_configs():
        if app_config.models_module is None:
            continue
        if verbosity >= 2:
            print("Running post-migrate handlers for application %s" % app_config.label)
        models.signals.post_migrate.send(
            sender=app_config,
            app_config=app_config,
            verbosity=verbosity,
            interactive=interactive,
            using=db)