diff --git a/.gitignore b/.gitignore
index 505a3b1..f1e7339 100644
--- a/.gitignore
+++ b/.gitignore
@@ -5,6 +5,7 @@ build/
 dist/
 wheels/
 *.egg-info
+.mypy_cache
 
 # Virtual environments
 .venv
diff --git a/pre-commit b/pre-commit
new file mode 100644
index 0000000..f7b3d81
--- /dev/null
+++ b/pre-commit
@@ -0,0 +1,5 @@
+set -o pipefail
+
+uv run mypy src/
+uv run ruff check "$@"
+git diff --exit-code
diff --git a/pyproject.toml b/pyproject.toml
index 223c779..a9f0e33 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -17,3 +17,31 @@ build-backend = "uv_build"
 dev = [
     "ruff>=0.15.1",
 ]
+
+[tool.mypy]
+disable_error_code = ["import-untyped"]
+
+[tool.ruff.lint]
+select = [
+    "E",    # standard style checks
+    "F",    # standard code checks
+    "FAST", # for fastAPI
+    "ANN",  # for annotations
+    "B",    # for common bug causes
+    "A",    # for builtin variable shadowing
+    "PIE",  # for unnecessary code
+    "SIM",  # for simplifications
+    "TID",  # for tidy improts
+    "TC" ,  # for type checking
+    #"ARG",   # for unused arguments
+    #"TD",   # checks for TODOs
+    "N",    # naming conventions
+    "W",    # whitespace warnings (use --fix)
+    "PLC",  # common conventions
+    "PLE",  # common errors
+    "PLR",  # common refactors
+    "PLW",  # common warnings
+    "UP",   # for old style
+    "RUF",  # other RUF specific rules
+]
+extend-ignore = ["TC", "PLR2004"]
\ No newline at end of file
diff --git a/src/fiber_package/base.py b/src/fiber_package/base.py
new file mode 100644
index 0000000..26dfda4
--- /dev/null
+++ b/src/fiber_package/base.py
@@ -0,0 +1,7 @@
+from pydantic import BaseModel, UUID7
+
+from .refs import CollectionName
+
+class FiberBaseModel(BaseModel):
+    uuid: UUID7
+    collection_name: CollectionName
diff --git a/src/fiber_package/groups/committees/committee.py b/src/fiber_package/committee/committee.py
similarity index 50%
rename from src/fiber_package/groups/committees/committee.py
rename to src/fiber_package/committee/committee.py
index 898da4f..4b57b21 100644
--- a/src/fiber_package/groups/committees/committee.py
+++ b/src/fiber_package/committee/committee.py
@@ -1,7 +1,13 @@
-from pydantic import BaseModel, model_validator
-from fiber_package.util import EntityList, GroupList, GroupRef
+from pydantic import model_validator, UUID7
 
-from typing import Self
+from fiber_package.refs import (
+    CommitteeEditionRef,
+    CommitteeEditionRefList,
+    EntityRefList
+)
+from fiber_package.base import FiberBaseModel
+
+from typing import Self, Literal
 from datetime import date
 
 from .enums import CommitteeType
@@ -12,31 +18,33 @@ Every committee has a list of editions.
 An edition is a definite group of members with functions and a timespan.
 '''
 
-class CommitteeModel(BaseModel):
-    full_name_EN: str
-    full_name_NL: str
-    description_EN: str = ""
-    description_NL: str = ""
+class CommitteeModel(FiberBaseModel):
+    collection_name: Literal["committee"] = "committee"
+
+    full_name_en: str
+    full_name_nl: str
+    description_en: str = ""
+    description_nl: str = ""
     committee_type: CommitteeType
 
     active: bool = True
 
-    editions: GroupList
-    current_edition: GroupRef | None
+    editions: CommitteeEditionRefList
+    current_edition: CommitteeEditionRef | None
 
     @model_validator(mode='after')
     def current_edition_in_editions(self) -> Self:
-        uuid = self.current_edition
-        if uuid is None:
+        if self.current_edition is None:
             return self
+        uuid = self.current_edition.uuid
         if uuid not in self.editions:
             raise ValueError("Current edition must be in list of editions")
         return self
 
-    # TODO... maybe require that an active board always has a current_edition that is non-empty?
+class CommitteeEditionModel(FiberBaseModel):
+    collection_name: Literal["committee_edition"] = "committee_edition"
 
-class CommitteeEditionModel(BaseModel):
-    members: EntityList
+    members: EntityRefList
     functions: dict[UUID7, str]
 
     start_date: date
@@ -44,7 +52,7 @@ class CommitteeEditionModel(BaseModel):
 
     @model_validator(mode='after')
     def functions_to_members_only(self) -> Self:
-        for uuid in self.functions.keys():
+        for uuid in self.functions:
             if uuid not in self.members:
-                raise ValueError("Cannot only assign functions to members")
-        return self
\ No newline at end of file
+                raise ValueError("Cannot only assign functions to committee members")
+        return self
diff --git a/src/fiber_package/groups/committees/enums.py b/src/fiber_package/committee/enums.py
similarity index 100%
rename from src/fiber_package/groups/committees/enums.py
rename to src/fiber_package/committee/enums.py
diff --git a/src/fiber_package/entities/system.py b/src/fiber_package/entities/system.py
deleted file mode 100644
index 80276af..0000000
--- a/src/fiber_package/entities/system.py
+++ /dev/null
@@ -1,6 +0,0 @@
-from pydantic import BaseModel
-
-from .submodels import LoginInfo, LoginInfoResponse
-
-class LoginModel(BaseModel):
-    login: LoginInfo | LoginInfoResponse
\ No newline at end of file
diff --git a/src/fiber_package/entities/__init__.py b/src/fiber_package/entity/__init__.py
similarity index 100%
rename from src/fiber_package/entities/__init__.py
rename to src/fiber_package/entity/__init__.py
diff --git a/src/fiber_package/entities/enums.py b/src/fiber_package/entity/enums.py
similarity index 95%
rename from src/fiber_package/entities/enums.py
rename to src/fiber_package/entity/enums.py
index 3d5ccc1..ba27d87 100644
--- a/src/fiber_package/entities/enums.py
+++ b/src/fiber_package/entity/enums.py
@@ -15,7 +15,7 @@ class Gender(StrEnum):
     nonbinary = "n"
 
 class HonoraryStatus(StrEnum):
-    merit = ("merit",)
+    merit = "merit"
     honorary = "honorary"
 
 class OnlineAlmanacField(StrEnum):
diff --git a/src/fiber_package/entity/login.py b/src/fiber_package/entity/login.py
new file mode 100644
index 0000000..f68b07a
--- /dev/null
+++ b/src/fiber_package/entity/login.py
@@ -0,0 +1,7 @@
+
+from .submodels import LoginInfo, LoginInfoResponse
+
+from fiber_package.base import FiberBaseModel
+
+class LoginModel(FiberBaseModel):
+    login: LoginInfo | LoginInfoResponse
diff --git a/src/fiber_package/entities/member.py b/src/fiber_package/entity/member.py
similarity index 79%
rename from src/fiber_package/entities/member.py
rename to src/fiber_package/entity/member.py
index 3499199..62ba032 100644
--- a/src/fiber_package/entities/member.py
+++ b/src/fiber_package/entity/member.py
@@ -1,13 +1,12 @@
-from typing import Literal
 from pydantic import Field
 
 from .non_members import FormerMemberModel
-from .enums import Membership, OnlineAlmanacField
+from .enums import OnlineAlmanacField
 from .submodels import Address, LoginInfo
 
 class MemberModel(FormerMemberModel):
     # requires that Membership equals member
-    membership: Literal[Membership.member] = Membership.member
+    # membership: Literal[Membership.member] = Membership.member
 
     login: LoginInfo
     address: Address
@@ -23,4 +22,4 @@ class MemberModel(FormerMemberModel):
     online_almanac_fields: list[OnlineAlmanacField] = Field(default_factory=list)
     # IT stuff
     shell: str = "/bin/bash"  # TODO: support other shells?
-    mail_token: str | None = None
\ No newline at end of file
+    mail_token: str | None = None
diff --git a/src/fiber_package/entities/non_members.py b/src/fiber_package/entity/non_members.py
similarity index 66%
rename from src/fiber_package/entities/non_members.py
rename to src/fiber_package/entity/non_members.py
index 7eb3856..d3a53ab 100644
--- a/src/fiber_package/entities/non_members.py
+++ b/src/fiber_package/entity/non_members.py
@@ -1,8 +1,6 @@
-from typing import Literal
-from pydantic import Field
 
 from .person import PersonModel
-from .enums import HonoraryStatus, Membership, Gender
+from .enums import HonoraryStatus, Gender
 
 class FormerMemberModel(PersonModel):
     title: str | None = None
@@ -13,12 +11,13 @@ class FormerMemberModel(PersonModel):
     # company emails?
 
 class AlumnModel(FormerMemberModel):
-    membership: Literal[Membership.alumn] = Membership.alumn
+    # membership: Literal[Membership.alumn] = Membership.alumn
 
     graduated: bool = False
     receive_spam_mail: bool = True
 
 class ExternalModel(PersonModel):
-    membership: Literal[Membership.extern] = Membership.extern
+    # membership: Literal[Membership.extern] = Membership.extern
+    pass
     # TODO... payment???
     # TODO require login info??
diff --git a/src/fiber_package/entities/person.py b/src/fiber_package/entity/person.py
similarity index 76%
rename from src/fiber_package/entities/person.py
rename to src/fiber_package/entity/person.py
index 0befc52..d8f4a2c 100644
--- a/src/fiber_package/entities/person.py
+++ b/src/fiber_package/entity/person.py
@@ -1,10 +1,13 @@
-from pydantic import BaseModel, model_validator, EmailStr
-from typing import Self
+from pydantic import model_validator, EmailStr
+from typing import Self, Literal
 
+from fiber_package.base import FiberBaseModel
 from .enums import Membership, LanguageOption
 from .submodels import Address
 
-class PersonModel(BaseModel):
+class PersonModel(FiberBaseModel):
+    collection_name: Literal["entity"] = "entity"
+
     membership: Membership
     first_name: str
     tussenvoegsels: str | None = None  # only in Dutch names
diff --git a/src/fiber_package/entities/submodels.py b/src/fiber_package/entity/submodels.py
similarity index 75%
rename from src/fiber_package/entities/submodels.py
rename to src/fiber_package/entity/submodels.py
index 37e52fb..8aefb84 100644
--- a/src/fiber_package/entities/submodels.py
+++ b/src/fiber_package/entity/submodels.py
@@ -1,17 +1,20 @@
 from dataclasses import dataclass, field
 from datetime import date
 
-# always use @dataclass with slots=True for better performance
+from pydantic import BaseModel
 
-@dataclass(slots=True)
-class LoginInfo:
+
+
+# always use @dataclass with slots=True for better performance
+# unless if you have unions like LoginInfo and Login...
+
+class LoginInfo(BaseModel):
     username: str
     hashed_password: str
     refresh_tokens: list[str] = field(default_factory=list)
     disabled: bool = False
 
-@dataclass(slots=True)
-class LoginInfoResponse:
+class LoginInfoResponse(BaseModel):
     username: str
 
 @dataclass(slots=True)
@@ -29,4 +32,4 @@ class PublicKey:
     name: str
     key_type: str
     key: str
-    expires: date
\ No newline at end of file
+    expires: date
diff --git a/src/fiber_package/groups/association.py b/src/fiber_package/groups/association.py
deleted file mode 100644
index d5fcea7..0000000
--- a/src/fiber_package/groups/association.py
+++ /dev/null
@@ -1,12 +0,0 @@
-from pydantic import BaseModel
-
-from fiber_package.util import EntityList, GroupList
-
-class AssociationModel(BaseModel):
-    full_name: str
-    members: EntityList
-    committees: GroupList
-    guilds: GroupList
-    # events: GroupList
-    # activities: GroupList
-    # mailings: GroupList
\ No newline at end of file
diff --git a/src/fiber_package/groups/root.py b/src/fiber_package/groups/root.py
deleted file mode 100644
index 2d3bfbd..0000000
--- a/src/fiber_package/groups/root.py
+++ /dev/null
@@ -1,6 +0,0 @@
-from pydantic import BaseModel
-from fiber_package.util import GroupList, EntityList
-
-class RootGroupModel(BaseModel):
-    associations: GroupList
-    sys_admins: EntityList
\ No newline at end of file
diff --git a/src/fiber_package/groups/committees/guild.py b/src/fiber_package/guild/guild.py
similarity index 53%
rename from src/fiber_package/groups/committees/guild.py
rename to src/fiber_package/guild/guild.py
index e21e107..03e2822 100644
--- a/src/fiber_package/groups/committees/guild.py
+++ b/src/fiber_package/guild/guild.py
@@ -1,9 +1,11 @@
-from pydantic import BaseModel, model_validator
-from fiber_package.util import EntityList
+from pydantic import model_validator
+from fiber_package.refs import EntityRefList, EntityRef
 
 from datetime import date
 
-from typing import Self
+from typing import Self, Literal
+
+from fiber_package.base import FiberBaseModel
 
 
 '''
@@ -13,16 +15,18 @@ They have a list of members.
 Removing old members from last_active is done in fiber
 '''
 
-class GuildModel(BaseModel):
-    full_name_EN: str
-    full_name_NL: str
-    description_EN: str
-    description_NL: str
+class GuildModel(FiberBaseModel):
+    collection_name: Literal["guild"] = "guild"
 
-    members: EntityList
-    previous_members: EntityList
+    full_name_en: str
+    full_name_nl: str
+    description_en: str
+    description_nl: str
 
-    guild_masters: EntityList
+    members: EntityRefList
+    previous_members: EntityRefList
+
+    guild_masters: EntityRefList
 
     last_active: dict[EntityRef, date]
 
@@ -30,7 +34,9 @@ class GuildModel(BaseModel):
     def current_members_not_previous(self) -> Self:
         for member in self.members:
             if member in self.previous_members:
-                raise ValueError("There can be no overlap between members and previous members!")
+                raise ValueError(
+                    "There can be no overlap between members and previous members!"
+                )
         return self
 
     @model_validator(mode='after')
@@ -39,4 +45,3 @@ class GuildModel(BaseModel):
             if master not in self.members:
                 raise ValueError("All guild masters must be members!")
         return self
-        
\ No newline at end of file
diff --git a/src/fiber_package/refs.py b/src/fiber_package/refs.py
new file mode 100644
index 0000000..3088c69
--- /dev/null
+++ b/src/fiber_package/refs.py
@@ -0,0 +1,42 @@
+from dataclasses import dataclass
+from pydantic import UUID7
+from typing import Literal, TypeVar
+from collections.abc import Iterator
+
+'''
+CollectionName provides the list of all allowed collections!
+'''
+CollectionName = Literal["system", "entity", "committee", "committee_edition", "guild"]
+
+C = TypeVar("C", bound=str)
+
+@dataclass(slots=True)
+class Ref[C: str]:
+    uuid: UUID7
+    collection_name: C
+
+@dataclass(slots=True)
+class RefList[C: str]:
+    uuids: list[UUID7]
+    collection_name: C
+
+    def __iter__(self) -> Iterator[UUID7]:
+        return iter(self.uuids)
+
+    def __contains__(self, item: UUID7) -> bool:
+        return item in self.uuids
+
+SystemRef = Ref[Literal["system"]]
+SystemRefList = RefList[Literal["system"]]
+
+EntityRef = Ref[Literal["entity"]]
+EntityRefList = RefList[Literal["entity"]]
+
+CommitteeRef = Ref[Literal["committee"]]
+CommitteeRefList = RefList[Literal["committee"]]
+
+CommitteeEditionRef = Ref[Literal["committee_edition"]]
+CommitteeEditionRefList = RefList[Literal["committee_edition"]]
+
+GuildRef = Ref[Literal["guild"]]
+GuildRefList = RefList[Literal["guild"]]
diff --git a/src/fiber_package/system/association.py b/src/fiber_package/system/association.py
new file mode 100644
index 0000000..857fcdf
--- /dev/null
+++ b/src/fiber_package/system/association.py
@@ -0,0 +1,15 @@
+from fiber_package.base import FiberBaseModel
+from fiber_package.refs import EntityRefList, CommitteeRefList, GuildRefList
+
+
+class AssociationModel(FiberBaseModel):
+    full_name: str
+    members: EntityRefList
+    committees: CommitteeRefList
+    guilds: GuildRefList
+
+
+    # TODO: also add former_members, alumns, etc... ?
+    # events: GroupList
+    # activities: GroupList
+    # mailings: GroupList
diff --git a/src/fiber_package/system/root.py b/src/fiber_package/system/root.py
new file mode 100644
index 0000000..7b8bd6c
--- /dev/null
+++ b/src/fiber_package/system/root.py
@@ -0,0 +1,6 @@
+from fiber_package.base import FiberBaseModel
+from fiber_package.refs import SystemRefList, EntityRefList
+
+class RootGroupModel(FiberBaseModel):
+    associations: SystemRefList
+    sys_admins: EntityRefList
diff --git a/src/fiber_package/util.py b/src/fiber_package/util.py
deleted file mode 100644
index f4b84c4..0000000
--- a/src/fiber_package/util.py
+++ /dev/null
@@ -1,9 +0,0 @@
-from typing import Annotated
-from pydantic import RootModel, Field, UUID7
-
-
-EntityList = Annotated[list[UUID7], Field(default_factory=list)]
-GroupList  = Annotated[list[UUID7], Field(default_factory=list)]
-
-EntityRef = UUID7
-GroupRef = UUID7
\ No newline at end of file