from __future__ import unicode_literals from collections import deque from django.db.migrations.state import ProjectState from django.utils.datastructures import OrderedSet class MigrationGraph(object): """ Represents the digraph of all migrations in a project. Each migration is a node, and each dependency is an edge. There are no implicit dependencies between numbered migrations - the numbering is merely a convention to aid file listing. Every new numbered migration has a declared dependency to the previous number, meaning that VCS branch merges can be detected and resolved. Migrations files can be marked as replacing another set of migrations - this is to support the "squash" feature. The graph handler isn't responsible for these; instead, the code to load them in here should examine the migration files and if the replaced migrations are all either unapplied or not present, it should ignore the replaced ones, load in just the replacing migration, and repoint any dependencies that pointed to the replaced migrations to point to the replacing one. A node should be a tuple: (app_path, migration_name). The tree special-cases things within an app - namely, root nodes and leaf nodes ignore dependencies to other apps. """ def __init__(self): self.nodes = {} self.dependencies = {} self.dependents = {} def add_node(self, node, implementation): self.nodes[node] = implementation def add_dependency(self, migration, child, parent): if child not in self.nodes: raise NodeNotFoundError( "Migration %s dependencies reference nonexistent child node %r" % (migration, child), child ) if parent not in self.nodes: raise NodeNotFoundError( "Migration %s dependencies reference nonexistent parent node %r" % (migration, parent), parent ) self.dependencies.setdefault(child, set()).add(parent) self.dependents.setdefault(parent, set()).add(child) def forwards_plan(self, node): """ Given a node, returns a list of which previous nodes (dependencies) must be applied, ending with the node itself. This is the list you would follow if applying the migrations to a database. """ if node not in self.nodes: raise NodeNotFoundError("Node %r not a valid node" % (node, ), node) return self.dfs(node, lambda x: self.dependencies.get(x, set())) def backwards_plan(self, node): """ Given a node, returns a list of which dependent nodes (dependencies) must be unapplied, ending with the node itself. This is the list you would follow if removing the migrations from a database. """ if node not in self.nodes: raise NodeNotFoundError("Node %r not a valid node" % (node, ), node) return self.dfs(node, lambda x: self.dependents.get(x, set())) def root_nodes(self, app=None): """ Returns all root nodes - that is, nodes with no dependencies inside their app. These are the starting point for an app. """ roots = set() for node in self.nodes: if (not any(key[0] == node[0] for key in self.dependencies.get(node, set())) and (not app or app == node[0])): roots.add(node) return sorted(roots) def leaf_nodes(self, app=None): """ Returns all leaf nodes - that is, nodes with no dependents in their app. These are the "most current" version of an app's schema. Having more than one per app is technically an error, but one that gets handled further up, in the interactive command - it's usually the result of a VCS merge and needs some user input. """ leaves = set() for node in self.nodes: if (not any(key[0] == node[0] for key in self.dependents.get(node, set())) and (not app or app == node[0])): leaves.add(node) return sorted(leaves) def ensure_not_cyclic(self, start, get_children): # Algo from GvR: # http://neopythonic.blogspot.co.uk/2009/01/detecting-cycles-in-directed-graph.html todo = set(self.nodes.keys()) while todo: node = todo.pop() stack = [node] while stack: top = stack[-1] for node in get_children(top): if node in stack: cycle = stack[stack.index(node):] raise CircularDependencyError(", ".join("%s.%s" % n for n in cycle)) if node in todo: stack.append(node) todo.remove(node) break else: node = stack.pop() def dfs(self, start, get_children): """ Iterative depth first search, for finding dependencies. """ self.ensure_not_cyclic(start, get_children) visited = deque() visited.append(start) stack = deque(sorted(get_children(start))) while stack: node = stack.popleft() visited.appendleft(node) children = sorted(get_children(node), reverse=True) # reverse sorting is needed because prepending using deque.extendleft # also effectively reverses values stack.extendleft(children) return list(OrderedSet(visited)) def __str__(self): return "Graph: %s nodes, %s edges" % ( len(self.nodes), sum(len(x) for x in self.dependencies.values()), ) def make_state(self, nodes=None, at_end=True, real_apps=None): """ Given a migration node or nodes, returns a complete ProjectState for it. If at_end is False, returns the state before the migration has run. If nodes is not provided, returns the overall most current project state. """ if nodes is None: nodes = list(self.leaf_nodes()) if len(nodes) == 0: return ProjectState() if not isinstance(nodes[0], tuple): nodes = [nodes] plan = [] for node in nodes: for migration in self.forwards_plan(node): if migration not in plan: if not at_end and migration in nodes: continue plan.append(migration) project_state = ProjectState(real_apps=real_apps) for node in plan: project_state = self.nodes[node].mutate_state(project_state) return project_state def __contains__(self, node): return node in self.nodes class CircularDependencyError(Exception): """ Raised when there's an impossible-to-resolve circular dependency. """ pass class NodeNotFoundError(LookupError): """ Raised when an attempt on a node is made that is not available in the graph. """ def __init__(self, message, node): self.message = message self.node = node def __str__(self): return self.message __unicode__ = __str__ def __repr__(self): return "NodeNotFoundError(%r)" % self.node