diff --git a/bigchaindb/backend/mongodb/schema.py b/bigchaindb/backend/mongodb/schema.py
index 527476f0..12b873e0 100644
--- a/bigchaindb/backend/mongodb/schema.py
+++ b/bigchaindb/backend/mongodb/schema.py
@@ -108,8 +108,8 @@ def create_votes_secondary_index(conn, dbname):
 def create_assets_secondary_index(conn, dbname):
     logger.info('Create `assets` secondary index.')
 
-    # is the first index redundant then?
-    # compound index to order votes by block id and node
+    # unique index on the id of the asset.
+    # the id is the txid of the transaction that created the asset
     conn.conn[dbname]['assets'].create_index('id',
                                              name='asset_id',
                                              unique=True)
diff --git a/bigchaindb/backend/query.py b/bigchaindb/backend/query.py
index 8f1325ef..5c37647c 100644
--- a/bigchaindb/backend/query.py
+++ b/bigchaindb/backend/query.py
@@ -213,13 +213,28 @@ def get_block(connection, block_id):
 
 @singledispatch
 def write_assets(connection, assets):
-    # TODO: write docstring
+    """Write a list of assets to the assets table.
+
+    Args:
+        assets (list): a list of assets to write.
+
+    Returns:
+        The database response.
+    """
     raise NotImplementedError
 
 
 @singledispatch
-def get_assets(connection, assets):
-    # TODO: write docstring
+def get_assets(connection, asset_ids):
+    """Get a list of assets from the assets table.
+
+    Args:
+        asset_ids (list): a of list of ids for the assets to be retrieved from
+        the database.
+
+    Returns:
+        assets (list): the list of returned assets.
+    """
     raise NotImplementedError
 
 
diff --git a/bigchaindb/models.py b/bigchaindb/models.py
index 81ca898c..53266930 100644
--- a/bigchaindb/models.py
+++ b/bigchaindb/models.py
@@ -315,13 +315,33 @@ class Block(object):
 
     @classmethod
     def from_db(cls, bigchain, block_dict):
+        """
+        Helper method that reconstructs a block_dict that was returned from
+        the database. If checks what asset_ids to retrieve, retrieves the
+        assets from the assets table and reconstructs the block.
+
+        Args:
+            bigchain (:class:`~bigchaindb.Bigchain`): An instance of Bigchain
+                used to perform database queries.
+
+        Returns:
+            :class:`~Block`
+
+        """
         asset_ids = cls.get_asset_ids(block_dict)
         assets = bigchain.get_assets(asset_ids)
         block_dict = cls.couple_assets(block_dict, assets)
         return cls.from_dict(block_dict)
 
     def decouple_assets(self):
-        # TODO: Write documentation
+        """
+        Extracts the assets from the `CREATE` transactions in the block.
+
+        Returns:
+            tuple: (assets, block) with the assets being a list of dicts and
+            the block being the dict of the block with no assets in the CREATE
+            transactions.
+        """
         block_dict = deepcopy(self.to_dict())
         assets = []
         for transaction in block_dict['block']['transactions']:
@@ -335,7 +355,20 @@ class Block(object):
 
     @staticmethod
     def couple_assets(block_dict, assets):
-        # TODO: Write docstring
+        """
+        Give a block_dict with not assets (as returned from a database call)
+        and a list of assets, reconstruct the original block by puting the
+        assets back into the `CREATE` transactions in the block.
+
+        Args:
+            block_dict (:obj:`dict`): The block dict as returned from a
+                database call.
+            assets (:obj:`list` of :obj:`dict`): A list of assets returned from
+                a database call.
+
+        Returns:
+            dict: The dict of the reconstructed block.
+        """
         # create a dict with {'<txid>': asset}
         assets = {asset.pop('id'): asset for asset in assets}
         # add the assets to the block transactions
@@ -348,7 +381,19 @@ class Block(object):
 
     @staticmethod
     def get_asset_ids(block_dict):
-        # TODO: Write docstring
+        """
+        Given a block_dict return all the asset_ids for that block (the txid
+        of CREATE transactions). Usefull to know which assets to retrieve
+        from the database to reconstruct the block.
+
+        Args:
+            block_dict (:obj:`dict`): The block dict as returned from a
+                database call.
+
+        Returns:
+            list: The list of asset_ids in the block.
+
+        """
         asset_ids = []
         for transaction in block_dict['block']['transactions']:
             if transaction['operation'] in [Transaction.CREATE,
diff --git a/tests/backend/mongodb/test_queries.py b/tests/backend/mongodb/test_queries.py
index 1363d9d7..c43c5fa4 100644
--- a/tests/backend/mongodb/test_queries.py
+++ b/tests/backend/mongodb/test_queries.py
@@ -1,4 +1,7 @@
+from copy import deepcopy
+
 import pytest
+import pymongo
 
 pytestmark = pytest.mark.bdb
 
@@ -418,3 +421,46 @@ def test_get_txids_filtered(signed_create_tx, signed_transfer_tx):
     # Test get by asset and TRANSFER
     txids = set(query.get_txids_filtered(conn, asset_id, Transaction.TRANSFER))
     assert txids == {signed_transfer_tx.id}
+
+
+def test_write_assets():
+    from bigchaindb.backend import connect, query
+    conn = connect()
+
+    assets = [
+        {'id': 1, 'data': '1'},
+        {'id': 2, 'data': '2'},
+        {'id': 3, 'data': '3'},
+        # Duplicated id. Should not be written to the database
+        {'id': 1, 'data': '1'},
+    ]
+
+    # write the assets
+    query.write_assets(conn, deepcopy(assets))
+
+    # check that 3 assets were written to the database
+    cursor = conn.db.assets.find({}, projection={'_id': False})\
+                           .sort('id', pymongo.ASCENDING)
+
+    assert cursor.count() == 3
+    assert list(cursor) == assets[:-1]
+
+
+def test_get_assets():
+    from bigchaindb.backend import connect, query
+    conn = connect()
+
+    assets = [
+        {'id': 1, 'data': '1'},
+        {'id': 2, 'data': '2'},
+        {'id': 3, 'data': '3'},
+    ]
+
+    # write the assets
+    conn.db.assets.insert_many(deepcopy(assets), ordered=False)
+
+    # read only 2 assets
+    cursor = query.get_assets(conn, [1, 3])
+
+    assert cursor.count() == 2
+    assert list(cursor.sort('id', pymongo.ASCENDING)) == assets[::2]
diff --git a/tests/test_block_model.py b/tests/test_block_model.py
index 6e559cb2..981a64d7 100644
--- a/tests/test_block_model.py
+++ b/tests/test_block_model.py
@@ -153,3 +153,136 @@ class TestBlockModel(object):
         block = b.create_block([tx, tx])
         with raises(DuplicateTransaction):
             block._validate_block(b)
+
+    def test_decouple_assets(self, b):
+        from bigchaindb.models import Block, Transaction
+
+        assets = [
+            {'msg': '1'},
+            {'msg': '2'},
+            {'msg': '3'},
+        ]
+
+        txs = []
+        # create 3 assets
+        for asset in assets:
+            tx = Transaction.create([b.me], [([b.me], 1)], asset=asset)
+            txs.append(tx)
+
+        # create a `TRANSFER` transaction.
+        # the asset in `TRANSFER` transactions is not extracted
+        tx = Transaction.transfer(txs[0].to_inputs(), [([b.me], 1)],
+                                  asset_id=txs[0].id)
+        txs.append(tx)
+
+        # create the block
+        block = Block(txs)
+        # decouple assets
+        assets_from_block, block_dict = block.decouple_assets()
+
+        assert len(assets_from_block) == 3
+        for i in range(3):
+            assert assets_from_block[i]['data'] == assets[i]
+            assert assets_from_block[i]['id'] == txs[i].id
+
+        # check the `TRANSFER` transaction was not changed
+        assert block.transactions[3].to_dict() == \
+            block_dict['block']['transactions'][3]
+
+    def test_couple_assets(self, b):
+        from bigchaindb.models import Block, Transaction
+
+        assets = [
+            {'msg': '1'},
+            {'msg': '2'},
+            {'msg': '3'},
+        ]
+
+        txs = []
+        # create 3 assets
+        for asset in assets:
+            tx = Transaction.create([b.me], [([b.me], 1)], asset=asset)
+            txs.append(tx)
+
+        # create a `TRANSFER` transaction.
+        # the asset in `TRANSFER` transactions is not extracted
+        tx = Transaction.transfer(txs[0].to_inputs(), [([b.me], 1)],
+                                  asset_id=txs[0].id)
+        txs.append(tx)
+
+        # create the block
+        block = Block(txs)
+        # decouple assets
+        assets_from_block, block_dict = block.decouple_assets()
+
+        # reconstruct the block
+        block_dict_reconstructed = Block.couple_assets(block_dict,
+                                                       assets_from_block)
+
+        # check that the reconstructed block is the as the original block
+        assert block == Block.from_dict(block_dict_reconstructed)
+
+    def test_get_asset_ids(self, b):
+        from bigchaindb.models import Block, Transaction
+
+        assets = [
+            {'msg': '1'},
+            {'msg': '2'},
+            {'msg': '3'},
+        ]
+
+        txs = []
+        # create 3 assets
+        for asset in assets:
+            tx = Transaction.create([b.me], [([b.me], 1)], asset=asset)
+            txs.append(tx)
+
+        # create a `TRANSFER` transaction.
+        # the asset in `TRANSFER` transactions is not extracted
+        tx = Transaction.transfer(txs[0].to_inputs(), [([b.me], 1)],
+                                  asset_id=txs[0].id)
+        txs.append(tx)
+
+        # create the block
+        block = Block(txs)
+        # decouple assets
+        assets_from_block, block_dict = block.decouple_assets()
+
+        # get the asset_ids and check that they are the same as the `CREATE`
+        # transactions
+        asset_ids = Block.get_asset_ids(block_dict)
+        assert asset_ids == [tx.id for tx in txs[:-1]]
+
+    def test_from_db(self, b):
+        from bigchaindb.models import Block, Transaction
+
+        assets = [
+            {'msg': '1'},
+            {'msg': '2'},
+            {'msg': '3'},
+        ]
+
+        txs = []
+        # create 3 assets
+        for asset in assets:
+            tx = Transaction.create([b.me], [([b.me], 1)], asset=asset)
+            txs.append(tx)
+
+        # create a `TRANSFER` transaction.
+        # the asset in `TRANSFER` transactions is not extracted
+        tx = Transaction.transfer(txs[0].to_inputs(), [([b.me], 1)],
+                                  asset_id=txs[0].id)
+        txs.append(tx)
+
+        # create the block
+        block = Block(txs)
+        # decouple assets
+        assets_from_block, block_dict = block.decouple_assets()
+
+        # write the assets and block separatedly
+        b.write_assets(assets_from_block)
+        b.write_block(block)
+
+        # check the reconstructed block is the same as the original block
+        block_from_db = Block.from_db(b, block_dict)
+        assert block == block_from_db